Often added to video file names.
Sometimes you can also see "VOSTFrench", probably to make it clearer for non-Frenchmen.
Just "VOST" can also be used if it's clear that they're French subtitles (and not for example English subtitles which would be "VOSTA": "Version Originale Sous-Titrée en Anglais").
In French movie theaters usually only "VO" (original version) is written to say that the movie isn't dubbed, since it's obvious that there are French subtitles. This is opposed to movies in "VF" ("version française", French version) which are dubbed.