Version: 5.6
public bool isFallback ;


If true, match this editor only if all non-fallback editors do not match. Defaults to false.

Unity does a two-pass match to hook up editors with inspected types. First the non-fallback editors are tested, and if none match, then the fallbacks are tested for a match. Setting this flag lets you set up a default editor for a given type, while still permitting another editor type to override it.