... I like your notation. N/(R/S) is more accurate. But ... possibility? Like for example, if R is the item that's ... from having written just N/R in the out group originally ...
It means from the total of all three subzones, you will have 3 R. However, we also know that we can't have R in z1. So, it's their sneaky way of saying 3 R in z2 and z3.