Just to clarify. MSC is the support side of Army medicine and with a few exceptions (such as medivac which has the same requirements as aviation), you assess as any other branch.
MC is the technical side of Army medicine, the doctors and nurses. You don't really "assess" for MC. If you go to medical/nursing school that is your branch.
Not entirely true...
MS branch has clinicians, medical researchers, medical scientists, administrators, and aviators. When competing for promotion, I'm going up against podiatrists, MEDEVAC pilots, logistics officers, etc.
AN branch are just nurses...not MC...
MC branch are just MDs and DOs.
PAs belong to SP along with dieticians, OTs, PTs
VCs are only veterinarians.
Every branch has a specific requirement within a specific AOC. Please see DA PAM 600-4.
As a 13 year MS officer, I've served in three AOCs within the MS branch: 70B (MEDO admin), 70D (IM/IT technical), and 67D (clinical/ behavioral science). All required a specific level of education, a specific degree, and experience. God help me I'm PCSing to Ft Sam to head up the Army's policy for addressing behavioral health support for future conflicts. So I guess I can check that box too.
Special branches (AMEDD, JAG, Chaplain) are...special. It's hard for others outside of the special branches to articulate what happens inside special branches.
If I find a 450lbs pediatric thoracic surgeon in Beverly Hills and convinced him to take a $750k pay cut to join the Army; he'd waddle into my clinic as an O-6 first day in the Army.
Welcome to my world...