The invasion of the EBC that crushed the Sentinels remained a trauma that would not go away any time soon to those of the Posterity of the Sentinels. The idea of underground bunkers were not alien to the Sentinels, and what turned out to be the Posterity were largely those who have huddled within the relative safety of the underground bases, even if they sometimes had to deal with low supplies. These people took to the underground shelters, and they would stay there until rescued by the allied forces of the Posterity under Charim and the TIA under Areus Megas. Ever since, the new human members of the Posterity grew attached to an idea of underground bases of operations, so that even if the installations above the ground were destroyed, one can rebuild underground and appear again once the enemy clears away.
Foundations are literally the foundation of a Posterity base in the Commoners' Legion, constructed from a Hub structure, whether it be the full Command Base or the Outpost forward bases. This system is an amalgation of the C&C style construction and the Halo War style construction, as the foundations are not necessarily attached to the hub itself, although it must be placed at a vicinity of the hub, and the foundations support various buildings that carry out a function to support the Posterity war effort in that the buildings are built underground, and are lifted out from the foundation, occupying the ground supported by the foundation.
Each hub can support only a limited number of foundations, and foundations outside the line of supply and line of command of a hub structure will not function. Foundations are however, resistant to most weapons, and will shrug off most AT and AP fire.