I'm aware this is not part of the public API, but this is used by several tools to check whether the SDK build is running or if another tool triggered avatar preprocessors (play mode testing with VRCFury).

The property is set for the first (current) platform as it is set by which triggers the first call to , but unlike the normal build process via , it is not set again before the call to (after ), that means that after a platform switch, the property value resets and remains set to , since further platform builds begin from .