The Reactor must be attached to a Barracks
, or Starport
. It is not built by a worker
, but is instead automatically built after being bought at the parent building. It gives the parent building double production slots, effectively increasing unit output by 100%. However, since this means that a Tech Lab
cannot be attached, only basic units can be produced.
Add-ons like the Reactor can be used by any Terran player once the attached building is destroyed or moved, but are automatically targeted by enemies just like all enemy buildings, so you'd have to be careful to not destroy them if you plan to make use of them.
The other type of building add-on is the Tech Lab, which allows the parent building to produce more advanced units. Only one add-on may be attached to a building at once.