A20 was the original ending level for Manhunt 2. It would have involved Danny having the decision to side with Dr. Whyte or Leo. If you side with Leo, you have to kill Dr. Whyte in her lab. If Danny sided with Dr. Whyte, Danny would have fought Leo in Danny's mind, like Release Therapy.
A20_E1 would have been the ending where Danny kills Dr. Whyte, and A20_E2 was the ending where Danny kills Leo in Dixmor Asylum.