Levels of Battletech
Field Manual: Homebrew
From Maxtech pg 6:
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All Battletech rules now carry a Level One, Level Two, or Level Three designation. Level One Battletech rules represent the basic level of play described in Battletech, Fourth Edition [ed. note this is the boxed board game, which may no longer be made], and use the technology available in 3025, including all 'Mechs and weapons described in Technical Readout: 3025. [ed. most players also group the lowtech vehicles into Level One, so most of Technical Readout: 3026 as well]

Level Two Battletech rules represent the rules used in most Battletech tournaments and MechForce-level competition. Level Two Battletech is defined by rules contained in BTC:RoW [ed. Battletech Companion: The Rules of Warfare has been replaced by the Battletech Master Rules or BMR], which includes all Level One rules plus the technology available in 3059 [ed. I believe this should now be consistent with the latest TROs which go into the upper 3060s]-the 'Mechs, vehicles, equipment, and other technology described in Technical Readouts 3026, 3050, 3055, and 3058 [ed. Also units from the official Record Sheets, and Technical Readouts 2750, 3060, Operation Phoenix, and the various Field Manuals. Please note I believe the including of some of the "equipment" found in the back of 3026 to be a unintentioned mistake as the rules weren't ever reprinted.].

Level Three Battletech play may include any of the optional rules presented in various Battletech Products [ed. generally this means Maxtech, Unbound (Mechwarrior Adventure), and the Tactical Handbook, though the equipment in Unbound and Tactical Handbook is poorly balanced and has been hotly debated, at times being excluded even from "Level Three" Battletech], as well as in the MechForce North America Quarterly magazine. These rules are always identified as Level Three. Players may use Level Three rules as they see fit. Generally, Level Three rules are not used in tournament play.

[ed. please note all editor notes *ed. were made by me the website guy to bring the exert up to pace despite the book's age]
So where does that leave us?
Since this website is in no way official (either as a Battletech product, nor as a division of MechForce North America Quarterly) that leaves the rules and equipment presented here as a "Level Four." These are house rules and follow most of what Maxtech says about Level Three rules, except: they aren't official. Also please note that players may wish to mix and match Level Three (or Level Four) rules and equipment, especially with technology base (year of play and official equipment available). This is because of the lore provided some of these optional rules and equipment may make more sense in a setting other than super high tech (read as: late 3060s), such as the Maxtech rules for Lowtech 'Mechs.
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