Subfactions is a mechanism that allows a single faction to have multiple overlapping definitions that allow the rules of what commands, units, buildings, skills, upgrades, morphs, etc. are available to change. Subfactions generally progress from the "base" subfaction (i.e., no "sub" in the faction) on to more advanced subfactions. This progression may be linear, or it may branch allowing multiple paths of progression. Some examples of similar mechanism in other RTS games are the progression to different "ages" in Age of Empires or Empire Earth.