I often find the term Alpine schistosity (schistosité alpine in French, scistosità alpina in Italian) in texts about Alpine geology. Chronologically ordered phases of Alpine schistosity producing different schistous foliations are named, in everything I have read, S$_1$, S$_2$ (and S$_3$).
I think that notations S$_1$, S$_2$, ..., S$_n$ may well refer to any ordered orogenetic phases producing schistosity in any rock, anywhere, at any time. Please correct me if I am wrong.
Nevertheless, I wonder whether, when talking about S$_1$ Alpine schistosity, or S$_2$ Alpine schistosity, one usually refers to a well defined temporal phase of Alpine orogeny producing a particular schistous foliation. Is that so?
Anyhow, I must remark that all schistosities that S$_1$ and S$_2$ in the geology of the Alps not necessarily refer to foliations generated during the Alpine orogeny, since for example in Briançonnais orthogneiss it is usual to distinguish a Silurian-Ordovician S$_1$ schistosity and a late Carboniferous S$_2$ schistosity, both generated before the Alpine orogeny. I thank you very much for any answer!
*Vv.Aa., Alpi liguri, edited by the Italian Geological Society.