The Engineer is a unit capable of capturing enemy buildings or repairing friendly buildings.
Repairing a friendly building makes the engineer vanish, and instantly restores all of the building's health.
Engineers are consumed when capturing buildings, after a short external wait. Before release 20190314, capturing an enemy building does not vanish the engineer, but it takes 15 seconds (30 seconds for Forward Command) to complete. [1]
Because of the capabilities of the engineer, it is possible for a Allied player to get Soviet items and vice versa.
This is a great tactic to use since you can get powerful Soviet tanks by taking over a Soviet war factory. The player can also take over an enemy Construction Yard and build the other teams buildings, such as tech centers to get GPS, or war factories to get enemy vehicles. An easy way to do this is to take over unguarded enemy buildings are attack only defenses leaving it unprotected. It is helpful to note that a building cannot be sold while being taken over by an engineer.
The engineer is not to be confused with the Mechanic.