when I ported the acs-kernel module, I was curious about the usage of groups, so I embarked on a fruitless search for documentation. Other than the basic creation of membership and composition relations, there doesn't seem to be any documentation.
I did however get some insights on how groups, relational segments, and relational constraints operate by playing around with the test code for acs-kernel. Look in openacs-4/packages/acs-kernel/sql/postgresq/test. The rel-constraints.sql file is especially instructive, once you get the hang of how things are supposed to work.
The main problem with figuring out the groups usage, is that terms like object_id_one, object_id_two and relational side one and relational side two are not very descriptive or intuitive.
