Currently, it looks like this (it may change) -
Mustering Tent -
T1 : Conscripts
T2 : Strelky
T3 : Guards
Support -
T1 : Sharpshooter, Tank Hunter
T2 : Heavy Mortar, ZiS-2
T3 : Medic Truck
Tank Hall -
T1 : T-70, T-90
T2 : T-34, SU-85
T3 : IS-2
Armoury Upgrades -
Requires Mustering Tent : Extra Rifles, Molotovs, Sturmovie Ingenery
Requires Support Barracks : Artillery Spotter, Man against Machine (extra PTRD), Phosphor Shells (Mortar)
Requires Tank Hall : T-34/85
Outpost is a buildable point that you can garrison and reinforce from... I don't think you can relate it to anything this is currently in game. Its like a field HQ that you don't upgrade and can't build from.