acs_rels is a generalized mapping table. It's similar to the mapping tables like user_group_map that we had in openacs 3.2.5, but it's generalized so that it can be used to describe any mapping relationship. It forms the basis of the composition and membership relations that are at the core of the new parties/groups system in openacs 4.x.
It is used in both the oracle and postgresql versions of the portrait upload, but since its use doesn't change when using postgresql, I just glossed over its usage in the content-repository document. If you study acs-subsite/www/user/portrait upload-2.tcl, you will see that a relation is created between the user and the image, so that it is possible to later retrieve a user's portrait if you know their user_id.
