Two points: we don't have roles in OpenACS, and privileges don't have fixed meanings.
No Roles: a role is a collection/group of rights that can be granted to a user, currently there is no way to group rights, they have to be granted one by one. Actually this is just done implicitly by each application. When I become a member of the main site, I implicitly can do a number of things. Bug tracker uses roles explicitly. I have used an owner_id in a data model to allow me to select rows. Query-Writer uses membership in a rel_segment to check if a user can assume a role (which is a list of actions on objects).
Privileges have no meaning: You can use the existance of any privilege for any purpose (as a developer).
