When I browse on-line bike shops disc brakes are listed as "front" or "rear" e.g. here. I have never noticed any difference between front and rear brakes - disc or V-brake, hydraulic or mechanic. Except the cable/tube length(but moat brakes are sold without attached cable/oil tube). So what is the difference between a front and rear disc brake, if any?
The length of the cable/hose and the braking lever - Left for front, Right for Rear. There really isn't any real difference. If you change out the cable/hose you can hook either up and they will work the same.
This is generally true for most brakes, the differences generally deal with bolt, spacer, and cable/hose length.