So I've talked about matrix attributes and proxy attributes on this blog before but maya actually has a much larger range of attribute types than most people realize. In fact I had asked my rigging supervisor if it was possible to have a nurbsSurfacee type input or output on a transform node, but he wasn't aware of any way to do that. But when I was checking something on the addAttr command documentation I found out that you have access to many more attribute types than Maya

Imagine you have a leg and you need to set up the IK/FK switch. If your rig is simple it can be annoying to have to set up an additional control just for the switching. It would easier if you could put the switch attribute on the actual FK or IK controls. But if we put it on the IK control then when we switch to FK mode that control won't be visible, and if we put the attribute on an FK control then we run into the same problem. If we could put it on both of them and somehow