A Displacement constraint offsets the applied faces from their original position by a given direction and distance. These field are often just left untouched (zero) in order to fix a face, which is assumed to be totally constrained by its surroundings.
In order to set up a well formed elastic case, the part must be constrained.
It is possible to let the part slide freely along the different coordinate axes by checking the free checkbox. This will disable input in the numeric field of the freed component direction, since the value in the field no longer contributes.
The displacement is given in millimeters (mm) or meters (m).