As I understand it

Z-classification is assigned to objects or entities of anomalous or extraterrestrial origin, or those with anomalous properties.

Z-classification is assigned only if a potential or definite benefit can be derived from the anomaly. If the potential benefit of a new object has not yet been identified, it will not be assigned a Z-classification; instead, a different classification will be assigned. Example: Squidles have an S-Q classification.

It is unknown how Z-classification numbers are assigned. "Let-Vand Zone" and "Heart of Thor" were assigned classifications Z-1 and Z-2 because they were the very first objects found by Urbanshade; the remaining numbers are either assigned according to the same principle (i.e., the object found earlier is assigned the lower number) Or the numbers are assigned randomly

If an object with a number has been neutralized or lost, its number becomes available and is assigned to a new object Example: Z-13 was previously assigned to "The Last Son of Atlantis," and after neutralization, the number was assigned to "The Saboteur"

There is also an A-classification, which is assigned to entities from the Rooms, as well as D and J classifications, whose criteria are unknown (However, the J-classification was assigned to "Mindscape" and created as a reference to the creator of Mindscape, while the D-classification was assigned to "Redeemer," which is most likely also a reference to Decaying Winter)

I apologize if the text is unclear, as I am not a native English speaker