The 12 series MOS in the Army represents all construction trade jobs. These jobs include: Plumbing, Electrical, Engineering, Firefighting.
Some examples of 12 series MOS jobs include:
12A Engineer Officer
12B Combat Engineer
12C Bridge Crewmember
12D Army Diver
12F Crane Operator
12G Quarrying Specialist
12H Construction Engineering Supervisor
12K Plumber
Combat engineers are trained to fight their way into and out of any terrain or environment. They can support front-line infantry when necessary. Combat engineers are trained soldiers that can operate firearms, but they primarily specialize in explosives.