Combat Medic is a base damage reduction while reviving teammates and is really only helpful to getting further skills. Quick Fix decreases the deployment time for first aid kits which can come in handy. Uppers (Ace), the main reason for this skill tree, adds a total of 10 extra first aid kits to your inventory. Further, they are auto used on downed allies within 5m.
Stable Shot is a base increase to weapon stability.