As this topic has been posted in the 'Arms, it would be fair to consider it in a context outside the standard 'Fox, Firmware, or Freeview sections (the OP could have helped by stating context). As far as I can see that means published guides such as Radio Times etc, and I'm pretty sure they don't use symbols or abbreviations without also publishing a key. However, surely it's obvious that (quoting from the Telegraph)
S = subtitles
R = repeat
and perhaps a little less obvious
AD = audio description
SL = sign language