The Core specification comes with the following XSD:
http://docs.oasis-open.org/ebxml-msg/ebms/v3.0/core/ebms-header-3_0-200704.xsd

For Part 2, we refactored this to allow more elements of the schema to be referenced from other schemas (such as the multihop schema):
http://docs.oasis-open.org/ebxml-msg/ebms/v3.0/part2/201004/ebms-header-3_0-200704_refactored.xsd

The headerExtension attribute group in the core schema references the S11:mustUnderstand and S12:mustUnderstand attributes.
The refactored schema has a more complete coverage of the SOAP attributes:

S11:encodingStyle
S11:actor

S12:encodingStyle
S12:role
S12:relay

Actor and role are used in the multihop module. If we are updating the Core schema XSD, I think we should include these in the revised Core schema.

Also see email at: https://lists.oasis-open.org/archives/ebxml-msg/201306/msg00001.html