Markings differ by manufacturer, but usually:
OFF = Off, not operating at all
STBY = Standby (powered up but not transmitting)
ON = On, but no altitude reporting. (Mode A / ALT RPTG OFF)
ALT / XPDR / XPNDR = On, with altitude reporting (Mode C)
TST = Test mode
TCAS equipped planes will have additional modes (TA, TA/RA, TFC)
Some planes have AUTO mode (switches between ground & air modes). In ground mode (GND), only responds to Mode S.
Ofter there are additional switches to select altitude source, left vs right transponder, etc.