Maybe we could define the axis about which it rotates. By default, it would be the y-axis, but like somebody was saying in regard to turrets on a mech's shoulders, you could have one tilted 30 degrees or so to one direction and it would rotate about that axis. I think the debate about naming different kinds of sub-units different things is a little limiting. Saying "we have x number of sub-units, and they are a, b, and c" lacks imagination. Somebody might come up with a whole new use for them that you didn't think of.
By the way, I know this is the MG board and all, but there's been a ticket for this subject in GAE for quite a while...