If you're talking about a small Mackie (or Behringer knockoff), I think Harvey is correct, the ST does mean "Studio", as in going to an amplifier for headphone or foldback monitoring for the musicians.
Well, if the "C-R", "ST", and "2TR" are all switches under the "Output" section, they're all "Stereo" outputs - and go to the "Control Room", "Studio", and a "2 Track Recorder" respectively.
My 32 channel Yamaha labels the stereo main output master fader as "ST"... but hey, its Yamaha - they aren't nice enough boards to use big words like "stereo"
Wow, I've been gone for a few days - can't believe there was more discussion on this! Anyway, for the record, these labels are on a little 10 or 12 track Yamaha MG mixer. And, as Harvey suggested, they are all stereo outputs, so "ST=Studio" makes a bit more sense. I was surprised that the manual didn't ever say what they mean. Anyway, thanks, guys.