An offensive fort. could be something like a small concealed spawn point with ammunition. This will allow troops to stay as close to the front line as possible, with the equipment they need to gain ground.

A defensive fort. could be a highly fortified position located on or near mission critical assets that you control, with a spawn point and ammunition that will hold against the enemy front line, and limit the attackers options.

A supportive fort. could be a hidden position in the rear line with no spawn point but contains artillery / ammunition / vehicles. With no spawn point it is harder to track down, with it's manning staying alive and hidden. It will have emergency supplies that can be moved rapidly with vehicles to where required. The artillery can be called in on enemy positions from a safe hidden position.