Evil J, aircraft can be identified in a plethora of ways. Sticking with SSR, there are basically 3 methods:
1. Observance of compliance with an instruction to set an assigned 4 digit code (ie "GABCD Sqk 1234").
2. Observance of a pre-assigned 4 digit code (ie, ac gets given sqk on taxy, the radar controller observes the assigned code on departure).
3. Observance of an "ident" after a pilot has been instructed to operate the ident facility (regardless of code).
In all these cases (and especially the third), controllers should, where possible, also ensure that the location of the observed code correlates with other radar or non-radar derived info. For example, if you sqk ident having informed the controller you are over the Forth Road Bridge but he/she sees the ident over Dundee they must make further efforts to positively identify you.
Finally, a controller may allocate the same sqk (from his allocated block) to any number of ac regardless of type of service. The only problem is that this will quickly lead to confusion!