[mglucido]

Within any entity's entry in the browser is the Origin. It's a handy way to get things lined up. I wanted a given part to stay aligned in a given plane so I constrained each entity XY plane to be mated.

It was important to leave the other dimension unconstrained because the part dynamically moves in the "real" world.

Unfortunately when I mated the two planes, the face of the part that had been facing down suddenly was facing up. I don't know why this happened yet but I thought constraining two parts in the assembly so they had parallel planes would do the trick. The offset field won't work as I understand it since it requires a specific measurement. The offset of my part moves with the dynamic movement of the robot.