Gundato: If I recall correctly, there is SR2: Reboot and SR2: Reloaded.
One of them (probably Reboot) is an expandalone that is available just about everywhere.
The other (probably Reloaded) is an actual expansion that is only really available (in English) from some distributors. This one needs the core SR2
Both contain the same contents when installed.
Reloaded is the base game
Reboot is the expansion (Standalone in most cases as far as I know, my Steam version is, so is the GG one despite saying it wasn't a while ago)
Revolution is an 'expansion' that was only ever released in Russia but they didn't have the money to translate it to English. There was community translation thing but it did introduce some crashes sadly.
However, no fear, they're making a Space Rangers HD version which I assume will include all of the above + widescreen patch. Should be out end of the year!