In the model the P to C reference is 1:*, that means that C refers to one P and this will allow to set the reference to P on C but not the other way round. So not on P to C. The reference selector allows the user to select one object and not many objects.
If you want to create a function in which you select a C record from P to remove this from the reference set the association should be from P to C as a *:1 like below.