Proving Ground (XCOM2)
The Proving grounds is akin to the Foundry in XCOM: Enemy Unknown (2012). The Proving Grounds has various projects that you can undertake that can severly increase the effectiveness of certain items, such as the medkit or armors and it is required to construct it in order to progress through story of the game. It is also the place where you can construct and research some of the most powerful late game equipment.
New to XCOM 2 are the Experimental Ammo and Grenades and later Experimental Armors and Heavy Weapons. These are semi-random projects that yield a randomly selected grenade/ammo-type/armor type etc that give various bonuses to a solider, such as increased aim or the ability to poison biological targets with shots.
Almost all Proving Ground projects require Elerium Cores, a fairly rare item that can be looted during mission.