What are all the K-Class scenarios, and what do they all mean? I don't know either, but here's a list of (hopefully) every K-Class mention in tales and items, as of October 2016.
So if you want to create a list of definitions, this would be the place to start. Enjoy.
AK-Class:
"The SK-Class dominance shift, the AK-class loss of consciousness scenario, and whatever other letters there are" in It's All True
BK-Class:
"«This is a BK-Class 'Total Buzzkill' Scenario.»" in Sparkle Spectacular
CK-Class:
"The Old Man and the CK-Class Reality Restructuring Event" in Crack Fiction Contest (ONE DAY ONLY!)
"Some kind of CK-Class Restructuring" in Girls' Night Out Dressing Up
"My unique character flaws may help to protect me from any resulting reality shifts, but a CK-Class restructuring is still a CK-Class restructuring" in Incident 239-B - Clef-Kondraki
"«Inform O5 Council that we've just had a CK-Class Restructuring Event,» he said" in Incident Zero - Part 2
""You're saying a CK-Class Restructuring Event was caused by a human cloning device" in Incident Zero - Part 2
"Let's, uh… call this a CK-Class event" in It's All True
"This is a visual representation of the CK-Class Restructuring, they told him" in Of Multiverses and Dandelion Wine
"These threatening items have been identified as capable of causing CK-Class Reconfiguration events of diverse impact in human societies and earthly life in general if allowed to reach Earth" in SCP-179 Sauelsuesor
"This document will tell you who you are: a necessary security measure to prevent CK-Class Reconfiguration events from leaving this site" in SCP-2330 A Convincing Whiteboard
"Any last vestiges of hope are crushed into tiny nubbins as subjects undergo a CK-Class Total Containment Failure, finally expelling the contents of their bowels in a magnificent riptide that could best be compared to a single, large leak in the Hoover Dam: an unfathomably vast expanse of liquid ejected at a rate fast enough to be upsetting, but not fast enough to be merciful" in SCP-666A½-J The Roaring Flames of Hell
"«Inanimate objects has been working on 120 potential XK-Class scenarios involving E-Class objects for the past three months; the thing back in October was just a nice distraction.»" in And I Feel Fine
DK-Class:
"However, the instant before the crucial moment of blessed release, SCP-666½-J triggers a DK-Class Dominance shift, seizing control over the subject's nether regions and causing a massive shutdown of all the subject's bodily exits" in SCP-666A½-J The Roaring Flames of Hell
EK-Class:
"Misuse of Stage Fright could lead to an EK-Class scenario - and we wouldn't be able to do a thing about it, because nobody would even realize it had occurred" in Stage Fright
FK-Class:
"Testing conducted in these areas suggests the presence of particle/anti-particle interactions most commonly found during FK-Class Temporal Misalignments" in SCP-2216 Peripheral Visions
IK-Class:
"The unrest and devastation likely as a result has the potential to escalate into an IK-Class event, which will severely compromise the Foundation's ability to keep other SCP objects under containment and protect the human race from their influence" in SCP-1238 Tunnelfish
LK-Class:
"Description: SCP-2690 is an unidentified object that initiates an LK-Class species transmutation event, classified SERAPHIM-1390, if not kept under constant study" in SCP-2690 Angel's Tongue
NK-Class:
"Based on this, it is estimated that the culmination of an NK-Class scenario would occur approximately four weeks post-event should full neutralization not be achieved" in SCP-2102 Got Shoggoth_
"Site-███ is subject to an ongoing process of mass body disposal to prevent an NK-Class scenario" in SCP-2288 Copy of A
OK-Class:
"Summary executions of your enemies, carried out by MTF Omega-7.5 («OK-Class Reunion Tour Scenario»)" in SCP-231-J 0.453592 Kilograms of Flesh
QK-Class:
"The momentum in SCP-2460 will propel it all the way back out of the center of the earth, back to the surface, where it will gravitationally oscillate back and forth, back and forth, rendering more and more of the Earth out of phase with itself like Swiss cheese until the entire planet collapses into a central superdense pile of untouchable rubble, constituting a QK-Class Quantum Degeneracy End-Of-The-World scenario." in SCP-2480 An Unfinished Ritual
RK-Class:
"Banning's logs about the severely deteriorated mental state of his daughter, it is well within the realm of possibility that this event could trigger an RK-Class «Reality Restructuring» scenario, by means of SCP-2464-2's out of control reality disruptions." in SCP-2464 Suspension
"After 15-20 minutes of the subject's intestines experiencing a level of containment rivaling that of SCP-106 (and involving substantially more screaming), the subject will experience a brief RK-Class Rapture Scenario, feeling a relaxation of the lower muscles, a wave of elation, and a fleeting hope that the worst has passed." in SCP-666A½-J The Roaring Flames of Hell
"The consequences of an expansion of SCP-816 beyond Site-P/██ are unknown, and could be, in a best-case scenario, harmless, to, in a worst-case scenario, an RK-Class Restructuring Scenario, where SCP-816 would out-compete biological life." in SCP-816 Darwin's Erector Set
SK-Class:
"The SK-Class dominance shift, the AK-class loss of consciousness scenario, and whatever other letters there are" in It's All True
"Any breach in SCP-2309 is to be reported immediately and considered a potential SK-Class dominance shift scenario" in SCP-2309 Iron Wall of Dhul-Qarnayn
"Because of the eschatological significance assigned to SCP-2309, and the existence of written and archeological evidence suggesting that SCP-2309 was constructed as a barrier against Sarkic armies, it is believed that the breach of SCP-2309, if and when it should take place, may constitute an SK-Class dominance shift scenario" in SCP-2309 Iron Wall of Dhul-Qarnayn
"A sudden SK-Class Scorched Earth Scenario completely razes the interior lining of the afflicted's digestive tract as unholy murderflame rages throughout in a demonic vortex with a temperature of roughly HOLY-CRAP-ON-A-CUPCAKE degrees Kelvin" in SCP-666A½-J The Roaring Flames of Hell
"It is likely that an 001-Apotheosis Event would result in a SK-Class «Broken Masquerade» scenario `HighLight`FOOTNOTE 2 `MainText`, and likely following that, an XK-class «End-of-the-World» scenario." in TwistedGears-Kaktus Proposal The Broken God
"SK-Class scenarios will automatically trigger a backup of all sensitive Foundation information to maximum security data servers within self-contained, «deep well» sites (currently 117, 118, and 119)" in TwistedGears-Kaktus Proposal The Broken God
TK-Class:
"Anomaly SCP-1297 represents a localized TK-Class Causal Restructuring Event, resulting in all temporally local causality (roughly the previous 20-50 years) being forced progressively further backward along our current world-line" in SCP-1297 A Jar of Toenails
UK-Class:
"MTF Gamma-4 («Blondebeard's Crew») is to be notified to adjust the trajectory of the anomaly as required, and to prevent it from entering the earth's atmosphere and causing a UK-Class Global Irradiation event." in SCP-2493 One Man's Garbage
"As the subject's ability to endure the emotional and physical trauma of the end-of-days inferno raging within their tenderest parts inevitably fails, SCP-666½-J will enter its final stage: a UK-Class Universal Collapse scenario, in which the subject experiences one final crescendo of gastric ragnarok followed by a blessed, merciful state of unconsciousness" in SCP-666A½-J The Roaring Flames of Hell
VK-Class:
"Their will folded an apocalyptic event into a VK-Class reality restructuring event, where the anomalous became normal, and everything right is wrong again." in lolFoundation Hub Page
"Addendum 001-B: VK-Class reality restructuring events designates an anomalous phenomena theorized to occur if all instances of SCP-001-K are released from their containment cycles" in Roget's Proposal Keter Duty
"Containment personnel are currently researching means to mitigate any possible detonation of SCP-1178 caused by SCP-1984, as to not cause the annihilation of SCP-001 and an immediate VK-Class end-of-the-world event." in Roget's Proposal Keter Duty
"Instances of SCP-001-K are not to be released from SCP-001, unless the hastening of a VK-Class reality restructuring event becomes a desired outcome" in Roget's Proposal Keter Duty
XK-Class:
"A few were still up, trying to figure out what, if any, potential XK-Class scenarios would play out tomorrow" in And I Feel Fine
"Just because the Foundation's scrambling all of its resources trying to find an XK-Class scenario doesn't mean it's gonna happen" in And I Feel Fine
"The whole of the site was too focused on potential XK-Class disasters to be worried about a simple bit of mistletoe" in And I Feel Fine
"XK-Class 'or bust', as it were" in Document 1780-WL
"I continue to read the report, and phrases like «XK-Class» and «Patros-Omega» come up, but «end of the world scenario» is pretty damned clear." in Euclidean Thinking
"At least it's not a fucking XK-Class event" in It's All True
"At the risk of causing an XK-Class End of the World Scenario, SCP-231-7 was relieved from Procedure 110-Montauk following the deaths of SCP-231-1 through -6, and was allowed to give birth." in New Job
"Perhaps rumours about an innocent young girl who was the victim of a satanic ritual and what we were forced to do to her to prevent an XK-Class End of the World Scenario" in New Job
"The object in question was a box wrapped in red paper and tied with a golden bow, that would only open at midnight Christmas morning, and would either release a harmless gift or an XK-Class «End of the World» scenario" in Reconciliation
"It is theorized that another breach of this magnitude or greater may cause several simultaneous XK-Class End of the World Scenarios due to containment breach of other known SCPs alone." in SCP-1297 A Jar of Toenails
"In the event that containment is breached, Overwatch is to be alerted to the possibility of an XK-Class end of the world scenario." in SCP-1591 Fallen Star
"Due to the non-zero likelihood of an XK-Class event, determination of this is a Class 1 research priority. In the event that the presence of SCP-1719-1 within the general population becomes a confirmed threat, the Foundation Ethics Committee has approved reclassifying SCP-1719 as Keter, and implementing Procedure Z-1719-T as a joint Foundation/Global Occult Coalition operation." in SCP-1719 The Harrison-Grey Effect
"If impact with Earth or orbital insertion occurs without proper response and containment by Foundation operatives, these items of interest may be capable of causing XK-Class end-of-the-world scenarios." in SCP-179 Sauelsuesor
"XK-Class Scenario hypothesized upon the release of the game." in SCP-1992-J Hype Train
"Imagine if it found the true horror of a nuclear holocaust or an XK-Class scenario" in SCP-2006 Too Spooky
"Addendum: 2217-Thaumiel: On 5/14/2019, O5 Command unanimously voted for temporary collaborative measures with GOI-004, in order to prevent an XK-Class Scenario" in SCP-2217 Hammer and Anvil
"As most of the recommended preparations overlap with or are identical to preparations already underway in case of numerous other XK-Class scenarios, minimal additional resources are required." in SCP-2317 A Door to Another World
"As records show that the sequence of ██████████ events shows an exponential increase in lethality, it can be extrapolated that failure of the final chain (SCP-2317-G), would result in an XK-Class End of the World Scenario." in SCP-2317 A Door to Another World
"Failure at any point to properly perform Procedure 220-Calabasas may result in an XK-Class End of the World Scenario" in SCP-2317 A Door to Another World
"An unidentified object, presumed to be of abnormal geometric origin, initiated an XK-Class Vacuum Decay within 2493'" in SCP-2493 One Man's Garbage
"XK-Class scenario - a doomsday conclusion without explanation" in SCP-2510 Our Broken Salvation
"On 03/06/████, a group called the Order of the White Sun attempted to use SCP-2975 as a ritual conduit to cause an XK-Class end of the world scenario" in SCP-2975 Another Sun
"SCP-2995 is positioned in the center of the three SCP-2995-1 instances, which form an equilateral triangle around the former. This alignment must be upheld at all costs. SCP-2995-1 must maintain a distance from one another of exactly 9.56 meters; to facilitate this, the instances are bound to one another by a series of carbon fiber-reinforced polymer supports. This arrangement must be monitored at all times, and it is imperative that any dislocation of SCP-2995-1 be responded to immediately. In the event of a possible containment breach, damage to SCP-2995-1, or movement by SCP-2995, Overwatch is to be alerted to the possibility of an XK-Class End of the World Scenario." in SCP-2995 Radiant Son
"Through [REDACTED], a limited amount of data on SCP-2995's physical properties is available. Presently, the object is believed to be a quark degenerate matter (possibly strange matter) star, making the anomaly's color a result of gravitational redshift. ‘HighLight`FOOTNOTE 1 `MainText` Relativistic physics dictate that a stellar body maintaining such a diminutive volume given its mass (approximately 3.14 x 1020kg, according to calculations of SCP-2995’s gravity within the threshold), is highly improbable; how SCP-2995 has achieved its size is unknown. By all accounts, SCP-2995 produces thermal energy, electromagnetic radiation, and gravitational force more than sufficient to constitute an XK-Class End of the World Scenario; these effects, however, are limited to the ellipsoidal space around the anomaly. The dimensions of the space are, in turn, determined by the precise placement of SCP-2995-1 around SCP-2995. It is presently unknown why some amount of visual radiation escapes" in SCP-2995 Radiant Son
"In the event of an XK-Class End-of-the-World scenario, or if the door closes during flight, resulting in [DATA EXPUNGED], [DATA EXPUNGED], or transfers hostile beings in large numbers, these devices are to be detonated if Procedure 600-Shoki cannot be enacted." in SCP-616 The Vessel and the Gate
"Due to [DATA EXPUNGED] and the damage done to ground-zero, SCP-923 is currently undergoing O5 review for its potential to cause an XK-Class ev" in SCP-923 A Useful Tool
YK-Class:
"This, on its own, constitutes a YK-Class Restructuring scenario every two weeks." in SCP-2123 I Accidentally the Whole Universe
"Due to the quantity of particle/anti-particle pairs, researchers project SCP-2216-3 events to irreversibly damage the quantum fabric which comprises our reality by the year 19██, spiraling us toward a YK-Class Reality Restructuring Event by the year 20██" in SCP-2216 Peripheral Visions
"The former case, as you might guess, should trigger a vacuum meta-stability event (by default a YK-Class Scenario)" in SCP-2400 Temporal Dilation Facility
"While the artifact is stable, it must remain at Site-64T until further notice, due to the ongoing risk of a YK-Class Scenario." in SCP-2400 Temporal Dilation Facility
"As indicated by SCP-2700-1, the device is currently armed and set to activate in 2234 (exactly three hundred years after its initial arming). As this would inherently trigger a breach of SCP-2700-Omega, current containment protocols must be completed prior to the set date to avert a YK-Class Event." in SCP-2700 Teleforce
"If SCP-2700-Omega were to escape the interior of SCP-2700-3, an inexorable chain-reaction would occur, converting the rest of the universe to this inverted entropic state. The eventuality of this scenario would be a YK-Class Entropic Annihilation Event, resulting in the reduction of the entire universe to an infinitely energetic singularity (ostensibly an inverted Big Bang)." in SCP-2700 Teleforce
"Personnel assigned to the containment of SCP-2700 are to investigate all available material related to the origin and function of SCP-2700. The primary objective in regards to the artifact's containment is to be its deactivation prior to the occurrence of a YK-Class Event. In light of the catastrophic severity a possible breach by SCP-2700 presents, Statute-30-A may be waived by personnel with 5/2700 Clearance: proposals for the cross-neutralization of SCP-2700 utilizing other SCP objects (including those of Keter class) may be presented to the O5 Council for evaluation." in SCP-2700 Teleforce
"SCP-2700 is to remain under constant surveillance, and all data regarding the state of SCP-2700-Omega is to be updated semi-hourly. Personnel tasked with accessing SCP-2700-1's operating system must adhere to a set of guidelines present in Document-SCP-2700-1. There is to be no direct interaction with SCP-2700-3 barring unanimous approval of the O5 Council; breaches of this protocol warrant immediate termination. Any change in the behavior of SCP-2700-Omega must be reported immediately, as such activity could result in a YK-Class Event." in SCP-2700 Teleforce
ZK-Class:
"«ZK-Class Reality Failure Scenario,» One said." in Incident Zero - Part 2
"Recorded transmissions from SCP-1142 indicate Dm-AX/15487-A is experiencing or has experienced a ZK-Class Reality Failure scenario." in SCP-1142 A Cry for Help
"Dropping The Load will result in a ZK-Class End of Reality scenario." in SCP-2612 The Weight of the World A Donkey and Its Cart
Other:
"It was every K-Class scenario" in It's All True
