stream? stream-name indexer? (is | isnt) external
  OR (for entity test)
  entity (is | isnt) external

This keyword tests to determine whether the object attached to a stream is external (an externally defined output stream). It returns true if the object is external, and false otherwise. Replacing is with isnt will reverse the results.

It can also be used as an entity test that returns true if the entity is an external entity. In this form, it is used primarily when testing the entities named by the values of entity or entities attributes. This test will always be true in an external-text-entity or external-data-entity rule.