As far as I can tell, it stands for "Super Terrific", "Splendid Tone" or perhaps "Solder Terminal".
But to my eye it's not important, and likely was to facilitate the requirement in layout software of never giving the same name/label to anything more than once.
In the PSU V3 store thread Jason had mentioned it could mean that it's a terminal used for stacking power supplies. FWIW, 1/4" PCB blade style tabs (Faston) fit the ST labelled positions perfectly. Of course, they also fit two of the larger V+-/++--/+++--- through hole terminals as well.
