mandatory attributes?

Hi,
I have a question. Is it mandatory for the XACML representation of any
access control scenario to have the names of the (primary) attributes
in Target as subject-id, resource-id and action-id or can I use custom
URIs like urn:my-namespace:subject-name,
urn:my-namespace:resource-name, urn:my-namespace:event-name,
respectively.

Thanks,
karthik