When you give a parameter to GetReplicaNumber(), you are specifying which level of parent to reach (I believe, mkselsey knows more and has answered this in this link).
If you don’t give it any arguments, it will give you the copy number.
So if you have a single mother object with say 10 detectors inside of it, then calling GetReplicaNumber() will give you the detector number (0-9). But calling GetReplicaNumber(1) will give you the number of the mother object, which will always be the same.
I have done parameterization, and GetRplicaNumber() without arguments returns a unique parameter index