Construction units are what actually gets your army up and running from scratch. The main units are called Engineers, which come in three different tech levels, differing for building speed and array of available building choices. So, a T3 Engineer can build T1 structures as a T1 Engineer, but much faster. Other construction units include the Armored Command Unit, which starts with a limited building choice (less than a T1 Engineer) but slightly higher building speed, and can be upgraded, and the Support Command Unit, which can also be upgraded and already can build T3 structures. T3 construction units, SCUs and (upgraded) ACUs can also build Experimental units.