Published in BattleTech Master Rules Revised Edition.
Rules Level 1
Published in BattleTech, Fourth Edition boxed set, and generally dealt with the Technology Level first introduced in the Age of War along with one piece of equipment developed in the Succession Wars Era. Rules Level 1 equipment is on the referenced page. Note Infantry and Vehicles had no place in Rules Level 1 Play.
Rules Level 2
Published in BattleTech Master Rules Revised Edition, and dealt with Standard and Advanced Rules. This book included Age of War, Succession Wars era equipment, and Technology introduced in Technical Readout: 2750 and Technical Readout: 3050. Rules Level 2 equipment is on the referenced page.
The Game Play and Construction Rules dealt with in BattleTech Master Rules Revised Edition constitute the level of game play that most BattleTech players experienced, adding Game play rules for Infantry and Battle Armor, as well as game play and construction rules for Combat Vehicles to the BattleMechs that were the only units in the Level 1 Rules Set.
Construction Rules for infantry Platoons were added in Combat Operations, the Classic BattleTech Role Playing Game supplement Classic BattleTech Companion added construction rules for Battle Armor. Construction and game play rules for Support Vehicles were added in Combat Equipment Guide as well as AeroSpace Fighters in AeroTech 2 Revised Edition.
Rules Level 3
Published in the Tactical Handbook, Maximum Tech, and the official MechForce North America Magazines (that FASA exercised some oversight over), as well as MechForce Worldwide publications (that FASA never exercised creative control over). Rules level Three constituted Rules expansions added to increase the realism of the BattleTech Game but were too complex for standard play. Note, This Rules Level did not (and was never intended to) include house rules.