Annotations

When you have the need for annotating entities (eg. adding extra fields whose data is stored locally), this can be done by assigning a separate entity type/bundle (for example, a separate content type). The annotation fields can be accessed directly from the external entity, just like you would do with regular Drupal fields. This is also a way to allow comments on external entities.

These (internal) annotation entities are stored in Drupal's default storage (eg. database). The external entity automatically inherits all fields & values from its associated annotation entity.

Annotations in combination with the Inline Entity Form module is a powerful combination.