What does "1S" "2S" mean next to game title.

I have searched and cannot find an answer so I joined here... some games have multiple versions of the same game with separate download links for each but the only difference is they'll say "1S" or "2S". For example, Astal for Saturn has a link to download Astal(1S), another that says Astal(2S) and Astal(3S). But I cannot find info on what these abbreviations mean.