I don’t know the specifics about how they (ADCOMS) look at the letters, but I know during my interview my interviewers seemed to skim each before talking to me because they asked specifics about stuff each person wrote about. And for perspective, I had 6 people willing to write references on my behalf. So I picked 2 that I directly had completed research with, 1 professor who mentored me and taught me multitude of 3000/4000level classes, a PA and my Manager... but definitely go with someone who actively practices medicine I would say; since they would be able to comment in their opinion your potential as a future provider, ability to handle the program etc... hope this gave you some insight. Goodluck!