This lets you plant a more powerful auto-turret which will detect and fire at any enemies. You cannot purchase this Ability without first purchasing Light Turret.
The Medium Turret is an upgrade to the Light Turret that can only be purchased once the latter has. It inflicts about as much damage as an assault rifle. Like the Light Turret, it scans the area and continuously fires on a target until he is incapacitated, at which point it will fire on another target in the area. The Medium Turret is a good defender, and is extremely deadly if supported by the deploying engineer. It is best used for supporting high fire areas and strategic locations, as well as defending command posts and objectives.