I've got a Part that is just a bunch of Planes, and I'm trying to attach some Parts.  When I pick a Face of the Part and do a Distance Mate (I want to adjust this later) it works fine and a can crank the Park back and forth by editing the Mate's spin box.

So save having to pick Faces I'd like to create named Planes from the Part's face, so I can just pick the Plane from Feature Tree. But when I did that