If this is the one that comes with three lenses, it's fixed to three distances that correspond to the lenses. You move the camera to and fro until your subject is aligned.
In the 1960s, Zeiss Ikon sold something called "Contameter,"
which shouldn't be confused with the other Contameters. It's an all-purpose rangefinder that can be used with the three close-up Proxars (although there were four available). Or it can be used as a regular rangefinder.