PC-DMIS uses the first two calibrated slots to calculate the "safety plane".
Assuming that X+ is the actual orientation of the probe changer, and if there are slots at different X values, the operator must calibrate as port 1 the port that sits behind (X = X1) and as port 2 the port that sits in front (X = X2), or vice-versa.
The value for the TCRackClearance registry entry is then added to X3 = MAX(X1, X2) to obtain the distance that the arm moves in +X (or -X) after and before performing the probe drop-off sequence.