The 2nd Squad adds another march slot in Top Force. Your first squad can stay on a gathering assignment while the second handles a different task. The construction requirements include Base level 10.
How the second march works
Each squad has its own assignment and selected heroes. A hero already marching cannot be changed, and a deployed squad must return before it can be sent again. An extra slot gives you another simultaneous assignment, provided you have heroes and units ready for it.
The Barracks supplies trained units. Your army's capacity is tracked through the Drill Grounds; adding a march slot is a separate part of army development.
What building levels change
Upgrading the 2nd Squad adds building power and a march-speed bonus. It does not add a third march slot. That comes from the separate 3rd Squad.
Unit capacity within the formation and the amount of resources it carries are separate stats too. Hero Squad Size concerns units, while Load Capacity concerns cargo. Squad 2 has its own research branch, with combat, gathering and load improvements for that squad.
For the difference between deployment slots, unit capacity and resource load, read Squads Explained.
