RE: [saml-dev] Why is ID of type ID, whereas InResponseTo is of type NCName? And why is AssertionIDRef of type NCName?

> If so, then why are they declared to be of different types?

IDness in XML is special and reserved for attributes that are intended to be
enforcably unique in a document or referenced by expressions like URI
fragments for signing, etc. Referring to an ID does not require those
properties.

As an example, one might have multiple AssertionIDRef elements in a document
pointing to the same value. You can't do that if the type is ID.

As Conor said, NCName is the same lexical space as ID, but doesn't have the
IDness property.

-- Scott