Forum OpenACS Development: Re: Howto: Expand and Use acs-subsite Based Member Roles
Group_rels and Group_type_rels just define a list of usable relationship types, mainly to build a UI where a user could pick the relationship type to use when adding a new user, or changing the type of a user.
Relational segments are a representation of all users which a certain relationship type to a group. So basically group_rels defines which relational segments you can add users to. It is not enforced, its just for convenience in building a user interface to add users to groups in a certain role.