Passage: Players collect three resources: manpower, munitions, and fuel. Manpower is necessary to produce all units. Munitions allows players to upgrade individual squads or vehicles and use special abilities. Fuel allows players to purchase tanks and other vehicles, build base buildings and acquire global upgrades. The player can decide, at a manpower cost, to place observation posts on any captured resource points in order to increase the point's production by 40% and make them more durable against enemy takeovers; this means sacrificing resources in the short-term for a greater long-term intake. Resource points must also be connected as any unconnected captured points can not produce resources.

Question: Production of units is only available through which resource?
manpower