Experimental units are the most advanced units available. They are incredibly powerful, and most have a mix of abilities that make them even more effective on the battlefield. Each faction has three experimental units, and they constitute some of the major faction differences.
Experimental units are often considered "tech 4". They are constructed not by factories but by tech 3 engineers, Sub-Command Units, and upgraded ACUs. The resource requirements for experimental units are extreme, so there must be a robust economy available to support the construction. They also take so much time to construct that a single engineer should never be used to construct them. Instead, as large a group as possible should be used to accelerate the construction.
For a list of all experimental units, see the experimental units category.