They do take a severity into account, they look at the amount of support which is required by the person which has been diagnosed.
Which according to DSM V either can be level 1 ("Requiring support"), level 2 ("Requiring substantial support") or level 3 ("Requiring very substantial support").
Which according to DSM V either can be level 1 ("Requiring support"), level 2 ("Requiring substantial support") or level 3 ("Requiring very substantial support").
As explained here:
https://www.autismspeaks.org/levels-of-autism