Would be cool to have a mechanic that allows you to replenish your squad members while in cap zones, or in a rearm zone near your spawn. I went a full match, 70+ kills, while single-manning a tank after an intense early fight.
Or wait, is that what reserves are? Have I been playing wrong this entire time? 
i am not 100% sure but I believe reserves mean you can use squads faster (at weaker strength), at least that’s sort of what happened in the pre-alpha, but they were specific to squads back then.
For me it appears the reserve function is just so you can reuse the equipment of the certain soldier without having to singlehandly unequip the guy yourself.
Sending to reserves/rear lines (bottom right there’s a button for that when you select a soldier that’s not in a squad) is removing your soldier to make space for new ones.
Soldiers not in your equipped squads of course do not take part in a battle