What is the function get_token_user_sub() supposed to do?

Question

I am currently building an OAuth 2.0 endpoint with authlib. At the stage of creating a token introspection endpoint, the official documentation suggests a function get_token_user_sub(token) in the method introspect_token(). What is it supposed to return? What is a "sub" in this context?

Answer 1

sub is subject in userinfo, usually it is the user ID.