You should look at the acs-relationships system. It is designed to define relationships between acs-objects. You can define a relationship type and attach attributes to the relationships.
See:
https://openacs.org/doc/sql/display-sql?url=acs-relationships-create.sql&package_key=acs-kernel
and
https://openacs.org/doc/openacs-4/object-system-design.html#objects-design-relation-types
I think the data model is more enlightening than the documentation.
