These are machinable stopper models which are especially suitable for machining of complex workpieces, or when extreme accuracy is needed and product-specific jaws are used for machining.
RMS-M has two extended jaws that are machinable. Furthermore, all combinations can be machined because these models are made of the relatively soft material (HRC 30, not hardened or nitrided). In the RMS-MS model, one jaw is of the "normal" length, and normally it is smooth. For machining purposes this model is slightly bigger in size.
Also many other variants of combinations can be made, so "x" can be =R ( serrated), V (vertical slot) etc.