You may use the Design Workshop screen to create new unit types. Whenever you create a unit using a weapon, armor, or chassis type which you have never used before, you must prototype the unit.
A prototype unit requires an extra +50% Minerals to build, though it receives a bonus +1 Morale upgrade in compensation since experimental weapons are often given to crack, well-trained troops.
Once you have prototyped a new unit, you may build that unit in the future at no additional cost.