Tagging for APM Entities

Hi we are creating/migrating Monitors need to Apply tags please clarify:

  1. What exactly is ENTITY_GUID
  2. How is it different from app id
  3. It is observed that few of the APM entities has it and the others did not have what is the rationale
  4. can’t we apply tags to APM entities which does not have ENTITY_GUID as the tagging NerdGraph GraphiQL API requires it