To the original poster, this is a bit like...
If you need to ask "how much?", you can't afford it
...or in other words...
If you don't understand it, then you haven't reached that level in the game yet.
These callsigns are mostly legitimately used by commercial operators for various reasons, the simplest being the likelihood of a last-minute airframe change. It's also helpful for some pilots (like me) who end up having their own callsign within an organisation (with that operator, regardless of what I'm flying, I have my own number, except on charters, where we use the charter trip number).
Some 'individuals' (and I use the word in its loosest possible sense to encompass 'operators', mostly at North Weald, who are playing at being fighter pilots) have adopted their own callsigns, and are only a letter of complaint or a 3113 (assuming my memory has accurately identified the new name for the 939) from a call from Enforcement.
Oh, and doubtless, someone will be along with the 'stupid formation' dit in a few posts from here.
If you're flying G-ABCD, and a grown-up hasn't told you otherwise; or you're not sure what a grown-up is; or don't know if you've met one (take special note if you fly from North Weald); then you should call yourself 'G-ABCD'.