Contact

Menu: Geometry->Create→Contact

Contact surfaces and volume are pseud-geometric entities that act as a link between a pair of 'similar entities'
Both entities must be on the same 3D location, or in different locations (separated volumes) with a solid-rigid movement with some distortion.

More than a geometrical entity could be considered as a kind of meshing criteria, with master and slave nodes connected by elements (with zero jacobian if master and slave are on the same location)

These special entities are useful to simulate a contact of two domains.