Add-on

Add-ons are terran auxiliary structures which are produced by and attached to a main structure capable of lifting off.

Buildings can leave behind or "acquire" add-ons through the use of lifting-off.

StarCraft
The command center can be attached to two add-ons, the comsat station and the nuclear silo. It can only be attached to one add-on at a time.

The factory can be attached to the machine shop, which is required to produce siege tanks and upgrade both vultures and the siege tanks.

The starport can be attached to the control tower, needed to produced advanced ships and holds upgrades for most terran ships.

The science facility can be attached to two add-ons, the physics lab and the covert ops. The former researches battlecruiser upgrades, the latter researches ghost upgrades.

StarCraft II
Main articles: Nuclear reactor and tech lab In StarCraft II, the terrans have only two add-ons, but they can be produced from the barracks,factory and starport. The reactor enables units to be produced more quickly, while the tech lab enables the creation of certain higher-tier units. Buildings cannot be attached to more than one add-on at a time.