Just keep in mind that security-role-ref does not make any sense for MDB and you generally won't specify it. The exam will not
test you with questions like 'What error occurs when security-role-ref is specified as XYZ for a MDB?' Most of the gray areas from the spec are carefully avoided/questioned in the exam. So don't worry.
SCJP, SCWCD, SCBCD, OCA J8