For 1.5.0, sid_ref_type was defined and was supposed to be applied to all elements and attributes where an SID was referenced, but it's currently used in the schema only for elements or attributes whose names contain "SID".
The spec says that the following (xxx) are of sidref_type, but they're currently all xs:NCName:
* <channel target=xxx >
* <param ref=xxx > (except not in render targets or kinematics)
* <instance_rigid_body body=xxx>
* <instance_rigid_constraint constraint=xxx>
* <bind target=xxx>
* <texenv sampler=xxx>