According to our knowledge base:

Subway is a scene group not yet confirmed (as in, i still don't have a screenshot of it to make sure it exists).
SSP is an abbreviation used by "Subway & Spreadpoint", a joint group from Subway and Spreadpoint groups, also not yet confirmed (both the group *and* their usage of SSP in any screen).
Spreadpoint is a known group which used "SP" as an abbreviation and is used in TOSEC for that too. All Amiga sets use this abbreviation except:
Kaiser II (1987)(Doerrschuck, Andreas)(de)(PD)[h Spreadpoint]" from Commodore Amiga - Games - Public Domain - [ADF], a set for our new renamer on amiga (Crashdisk) to fix

There are also 21 sets published by Spreadpoint, in these cases we tend to use the full name in order to avoid confusions. They are all demos and diskmags.