AFAIK there is no definition of or guidance on what represents 'supervision'
Sadly that seems to be the case. I've been 'supervised' from the end of a phone line 'call me if you've got a problem'. Personally, now I'm supervising, I think it OUGHT to mean being on-site, maybe having a sneaky over-hear of brief/de-briefings. If they're away on a navex, I think it's OK to be in the circuit but never far away from a phone. There's a fine line between keeping a fatherly eye on things and interfering but you should always 'be there' for them.
TOO