Right. So, just to be clear:

1. The "Members Document" is a "child document" that contains all the members of the parent resource (group or content)
2. The "Memberships Document" is a "child document" contains all the groups of which the parent resource is a member

So in that regard, the terminology is consistent and I think the naming of the documents are OK.

But, to query the content that is in my library, I query on the Members Document ("Give me the content of which I am a member"). And then to get all the members of a group, I search the Memberships Document ("Give me all the users and groups that have this group as a direct membership").

As you saw, to query "all my memberships", these documents cannot be used because of the indirect nature of that query. So I use getPrincipalMemberships to query by the full group hierarchy.

Hope that helps!