obligation
Both elements are identical in structure and content but differ in intent: the PEP must comply with an obligation while it may comply with an advice. To simplify the language we could keep obligation and add a boolean flag that tells the PEP whether it has to understand and process the obligation.
Switching from 2 elements (advice and obligation) to a single one with a new parameter (the flag) is too much work for now and not worth it. We can revisit later.
XACML and ALFA both define: