I hope I get your question correctly. The AAA group tag is local to the AAA Client and has nothing to do with the AAA Server (e.g. ACS). It is meant to group more than one TACACS/RADIUS server.
Proxy Distribution Table is used when you have Multiple ACS servers and you want to route incoming AAA requests to particular server(s) based on pre-defined criteria. Like user1@NY should be redirected to the NewYork ACS.
Regards
Farrukh