Interesting article on the difference between train supervisors and guards at:-
http://trains-today.blogspot.com/2016/07/comment-southern-strikes-continue.html
Essentially it all seems to come down to guards also operating the doors as well as patrolling the train and selling tickets and train supervisors not operating the doors or monitoring the moving off from the platform as that role now transfers to the Driver Only Operator (DOO). The argument is clearly about the fact that with a DOO nobody is monitoring the safety of passengers on the platform as the train moves off.
On my local Mole Valley line there is still always a guard between Dorking and Horsham station to cover Holmwood, Ockley and Warnham stations (which are all totally unmanned) but now only a DOO operator between Dorking and London Victoria most of the time.
Previously the guard used to also always continue as far as Epsom, principally to cover Boxhill & Westhumble (also totally unmanned) but now they mainly get on and off at Dorking station and don't travel further north. I don't know why a DOO is now considered safe at Boxhill & Westhumble but I think it may be because the platform has been extended so that it is now long enough for a maximum length 10 carriage train to open all doors, whereas the carriage is less than 8 carriages long at Holmwood, Ockley and Warnham. So I presume that the role of the guard at these three stations is to make sure that doors are locked out in the carriages where there is no platform and/or to take appropriate action if the doors in those carriages ever did inadvertently open at those three stations?
So is the argument about guards all about whether or not they are needed at stations that don't have platforms that cover all the carriages on the train? Clearly with reliable enough and properly tested software and adequate camera coverage ] there would seem no real reason why a guard is needed at those stations either as the DOO could also take the appropriate action and call the emergency service if the doors ever opened in a carriage where they shouldn't do at a station with a short platform. Of course the other argument of the unions is that it is less safe if there isn't a guard watching the train as it moves off from the platform but as all doors are now automatic and can't be opened manually from the outside (by ordinary passengers) surely as long as the DOO checks no one is stuck in the doors before moving off it is safe. The main theoretical safety danger would appear to be with someone standing on the platform who still has some piece of clothing or a thin strap stuck between the closed passenger doors (which the door closure sensors I suppose may not pick up)? Other tragedies such as children or youths (especially) as well as some adults running across the track at unmanned foot crossings or only part barriered level crossings and being killed or injured can't be stopped even by a guard.
Of course the other argument by the unions is really about protecting jobs (as some routes already don't need train supervisors in place of guards) and about protecting the pay level for the guard job (as a train supervisor is paid less than a guard it would appear)?