Difference between externalRef and include

The RELAX NG Specification says that the resource behind URI passed as argument to 'externalRef'
must much the syntax for pattern, while one passed to 'include' must match the syntax for grammar.

The Relax NG Compact Syntax says that both should reference a grammar in the compact syntax.

The latter makes me think that a file is either sintactically valid for both referencing from externalRef
and include or invalid for both contexts. Both Jing and nXML disagree with me.

Further on, jing parses contents of an external file passed to externalRef as a grammar,
but refuses to accept the same file if passed to include.

David Tolpin