AHow do people get all these extra bits of diagnosis?

Reading through various threads on here over the 3 months or so that I've been here, people say they've got things like high masking and all sorts of other things added to a diagnosis of autism. How do you know? Do these extra bits help you understand yourself better, do you get more help?

I just thought you got diagnosed as autistic and maybe one other thing like ADHD, but it would seem not, where does one go to get this sort of additional information in the UK?