Two entirely different things. The rear detachment First Sergeant, is someone is assigned in such a role, is the senior noncommissioned officer of that detachment, whereas the commander is the one actually in charge - typically, this will be a commissioned officer, although it is possible that an NCO could be tasked to fulfill the role.
captain
FRG
FRG
assignment, deployment
senior nco
Commissioned Officer
Senior NCO
Senior NCO
brig. general
field grade officer
assignment, deployment
Command and Control