They're known as models, face-claims, portrayers and more. But it all means the same thing: the real-life person who's pictures you are using for a character. You may not use someone who's already being used. You many not reserve more than 2 models at any given time. You can reserve models for 5 weeks, and if you don't use it by then, others can use it. And models cannot be a main character from Doctor Who (or main characters of a single episode).