So I finally got the mortar dudes but how to get the slot ???
Ok when I click on the “slot is locked area” it reads basically to go to Squad Upgrades but when going there the upgrades for the Mortar man do not allow for an extra slot to be unlocked so how does this slot get unlocked … Thanks …
What slot do you mean?
Edit: sorry I understand what you mean now.
They can not get that slot unlocked, for some reason that is unknown to me mortar dudes cant have backpacks.
Edit edit: Shit even after trying to undestand I didnt xD
are you like blind? 
You have several branches in the upgrade tab.
Those are:
-Squad, everything that effects the squad direclty i.e squad size, squad exp, squad composition
-Personal, everything that effects the main Soldier Class of the selected squad. In your case, the Mortarmen. There you will also find slot unlocks for sidearms, backpacks, 2nd gun slot, higher soldier rank and so on, wich is what you are looking for there.
-Workshop, everything to do with the corresponding equipment or rather, weapon. There you can upgrade for max rank Weapons, better disassemble and lower modification cost.
ok thanks I guess its just a pistol slot thanks for taking the time to post a pic and for the other lengthy replies. If one looks at the 16th Inf regiment the Big Red One those guys get two slots that are initially locked so they can have a pistol and then a small backpack, grenade pouch, ammo pouch etc. Glad to know its just a pistol slot I will not be fooling with it.