The gauging fixtures are manual measuring stations customized for the part to be checked.
The fixtures may look like stations inside which the part is carried (the so-called dog houses); this happens for instance with box-shaped parts (engine block, head, gear case and differential case, pump bodies) to check the presence of holes, their location and other. Once the part is referred on pins, and using the same machine tool references, the holes to be checked are measured either by attributes using pins (functional checks) or by variables using mechanical or electronic gauges like dials, pencil probes or electronic plugs (dimensional and geometric checks).
Other fixtures look like tools that are seized and then carried onto the part (the so-called layon gages); the checks performed are the same described above.
For further information please consult the page on gauging fixtures (dog houses and layon gauges) in the MG site.