The Infantry portal in Natural Selection 2 is utilized by marines to warp into combat upon death. The structure can be destroyed by aliens, crippling the spawn rate of marines.
Usage In game
The Infantry Portal (Generally called the "IP" in game) spawns the marines during general play. During prolonged or defensive games, it is wise to build more than one, but more on that later. It is, second to the command chair, one of the most important structures in the game. Without it, Marines cannot spawn in upon death, and effectively lose.
Upon dying, a player must wait several seconds before the portal "constructs" a new marine. According to the lore, it doesn't "rebuild" your character, merely warps in a marine from another location. Presumably one not under attack by aliens. If several marines die at once (Oftentimes the case after a large skirmish) then the portal will queue up the marines, warping them in one at a time. Marines cannot be killed while warping in, but upon fully spawning, become fair game. This can mean before you even have your LMG locked and loaded, a skulk camping the portal could have easily murdered you.