The game's popularity spawned several variants and expansions to the core system, including "CityTech" which fleshed out urban operations, infantry, and vehicle combat, "AeroTech" which focused on air and space-based operations, and "BattleSpace" which detailed large spacecraft combat. FASA also published numerous sourcebooks, known as Technical Readouts, which featured specifications for new combat units that players could select from. However, despite the large number of such pre-designed BattleMechs, vehicles, aerospace units and other military hardware, the creators also established a system of custom design rules, enabling players to generate their own units and field them in combat. In addition to game rule books, FASA published several background books detailing the history, political and social structures of various factions in the game, including all five Great Houses of the inner sphere, ComStar, the Periphery States and the fallen Star League.