XACML Meta Data Placeholders
version single selection from published XACML versions (although this raises some interesting versioning issues since this is new as of XACML 3.0, however I guess it could be possible to provide this information without full compliance with v3 as doing so would not invalidate support of an earlier version).
RealtimeAttributeEvaluation Boolean True|False
PIPURI Policy source (suggested by Rich)
Capabilities Which optional features, extensions and profiles the PDP implements.