Its not to do with being employed on a temporary basis at all! The jobs secure despite what the last posts have stated, you could be supernumery for days up to over a year depending
Basically as a supernumery you are outside the roster as theres not a spot ready for you yet. You still turn up every day and act as a spare (however the rostered spares and standby have to used before you are). You may ask to go and assist the rostered guard with tickets etc but that isn't always allowed. You will go into the link / roster when someone leaves for any reason and its your turn to enter (there may be a number of supernumerys)
I did say it was a potentially temporary job. I didn't say it was definitely temporary.