The editor number is a value between 1 and 32767 that is used by level editors to identify things placed in a map. In the DECORATE format specification, this value is listed at the end of an actor's declaration, before the actor block begins. Internally, a value of -1 for the editor number means that the actor cannot be placed in a map.
The editor number is, along with the SpawnID, filtered through the Game property to avoid conflicts. There is a list of standard editor numbers for reference, giving the editor numbers of all standard actors.