Artifact Format

SAML 2.0 Binding document  ("Artifact Format" - line 1046) defines artifact to be:

Artifact = [TypeCode(2) EndPointIndex(2) SourceID(20) MessageHandle(20)]

Length is denoted in brackets.

Does  SAML 2.0 compliant server MUST follow the above format?

How can I extend it?

Artifact = [TypeCode(2) EndPointIndex(2) SourceID(20) MessageHandle(20) FOOBAR(nn)]

Is this allowed? Any issue/restriction/issues? etc.

Thanks,

/Prasanta