If you select a 2D object while in 3D mode... the 2D objects Workplane can change while moving. If you get this message selecting a 2D object while in 2D mode, it means that the object is on a different Workplane than the one in use, and the Workplane will change to match the object.
So... in a nutshell.
1. In 3D selector mode... a 2D object will/can change Workplanes
2. In 2D mode... the Workplane will change to match the object.
Hell I don't know!