Note that the S$_{n}$ terms are used to describe schistosities. These are visual features observed in petrographic examination of rocks under the microscope. These are commonly open to interpretation and debate. You have to distinguish them from the D$_{n}$ events (deformation). S$_{n}$ features are usually used as a proxy for D$_{n}$ events, but it's not a one-to-one relationships. Let's say you know that 3 deformation events occurred in a rock. However, you can only see two schistosities. How do you call them? S$_1$ and S$_2$? What if you know that the first schistosity was erased? Do you use S$_2$ and S$_3$ then?

Again, these terms are used loosely and they may not be correlated between different areas and with time, unless mentioned explicitly in the study you are reading.