If the association from A to B is a reference set or a 1 : 0 reference, the constraints possibly apply to multiple objects (depending on the direction in which the association is traversed). In this case, there is a very distinct difference between the two constraints:
ReferenceA_B != [%CurrentObject%]means "There is at least one object that is not the current object" (which is true in most cases),
not(ReferenceA_B = [%CurrentObject%])means "None of the objects is the current object".
So in the case of multiple objects, the choice between these constraints depends on what you want to achieve.