Advice needed

My son has just turned 18 and I have received a transition form to fill in.It’s the one with a series of questions and the R S G boxes alongside.Can someone please just explain what the difference is,between ready and go.I’ve taken it to his GP and she isn’t sure either.
Many thanks in advance for any replies and i’m sure the penny will drop when it’s explained.

Who issues this Transition Form? They should explain what the letters mean on the form with an accompanying form.