Yeah, the X-File fandom was quite specific as well. It was it's own category. The assumed base state was friendship. If it went beyond friendship it would then be classified:

There was R -- which denoted romance
there was UST -- which denoted "unresolved sexual tension"
there was RST -- which was "resolved sexual tension"
there was PWP -- Plot? What, plot? (aka Porn without Plot)


Therefore you could get M/S -- which was friendship. Or MSR -- Mulder/Scully romance