Rob
Ansys Employee

The surface pair coincide where they overlap. If the two faces are different sizes then there will be an area on one, the other, or both that doesn't coincide. Equally, if there's a gap between the faces they neither coincide nor overlap. The English language is wonderful.... 

By moving I mean bits of solid sliding/rotating relative to another part. If the geometry is fixed then nothing is moving. The fluid is flowing, but the mesh/geometry isn't.