The Master Unit List has been running in one form or another for well over a decade. When we rebuilt it, we inherited a database that had been added to, corrected, and worked around by volunteers across most of that time — 8,700+ units, 138,000+ unit availability records, and a lot of decisions that made perfect sense when they were made, but didn’t necessarily reflect what was needed now.
This is a short rundown of how faction data is actually organized now. It’s more detailed than most people need, but worthwhile because a few of these choices explain behavior you’ll notice on the site.
What we inherited
The old database held more than anyone expected. Alongside the units and their availability, there was a whole layer of production geography that had been quietly accumulating without ever surfacing in the interface — but more on that later.
There was also a lot of accumulated compromise. A single generic table held a dozen unrelated vocabularies. Abilities were stored as raw text strings. And two fields — Rules Levels and Technology Levels — had turned into more of a hindrance than a help, so neither made the trip to the new MUL.
The first decision that shaped everything else
Availability is a matrix, not a date range.
It would have been simpler to give each unit a “from year” and a “to year” and be done. We didn’t, because that isn’t how BattleTech works. A unit isn’t uniformly available across the Inner Sphere — it’s available to particular factions, in particular eras, and those two things move independently. A design common in one realm can be unheard of in another for a century.
So the core of the MUL is a matrix: faction × era × unit. Every “can the Draconis Combine field this in 3025?” question is a direct lookup rather than a calculation. It costs more to store and vastly more to curate — every one of those rows is a decision someone made — but it’s the only structure that tells the truth about a unit’s availability.
Where units are built
Since the next search funnel is almost ready to push, I thought I’d include this information as well. Production geography got the same treatment as the unit matrix. Unit manufacturers became their own thing, and took the 1,000+ companies— which is a different question from the political factions in its own direction. Factories belong to a manufacturer and sit on a planet, with opening and closing years dictated by the era.
The part I’m happiest with: a factory doesn’t record which faction it belongs to. It records where it is. The producing faction is derived from who controlled that planet in the year the unit was produced.
That sounds like extra work, and it is. But a factory’s allegiance isn’t a fact about the factory — it’s a fact about the planet and the border it happens to sit inside, and borders move. Storing the owner would mean re-editing thousands of records every time a world changed hands. Storing the planet means BattleTech history does the work.
How factions are organized
I think this is where the new model earns its keep and offers us a lot of flexibility moving forward, because “faction” covers several different things.
Every faction sits in a parent group — Inner Sphere, Clan, or Periphery. That never changes — and remains a holdover from the original MUL.
Below that is a child group that is era-scoped — Major Powers, Minor Powers, Homeworld Clans, Inner Sphere Clans. Clan Wolf is a Homeworld Clan in one era and an Inner Sphere Clan in another. Rather than pick one and be wrong half the time, the grouping carries its own history based on era. It’s a subtle change, but accurately reflects faction changes in BattleTech.
Bands: sub-commands and quality tiers
Next up is the band. A band is a named, filterable slice of a single faction’s unit list. Bands come in two flavors.
Quality bands are rarity tiers — Royal, Front Line, Second Line, Solahma. A quality band describes how good the equipment is rather than who in the faction carries it, and they’re what the Generated Random Assignment Tables read when deciding how likely you are to roll something. Selecting one narrows the pool you’re sampling the RAT from.
Formation bands are sub-commands — the Shadow Divisions, the Protectorate Militia, and similar groupings are formation bands. A sub-command is a real formation with a distinct equipment profile that doesn’t warrant being modeled as a separate power. The sub-command gets its own tab on the faction page, drawing from that faction’s units, tagged to show what the formation actually fielded.
Two ideas to keep in mind when looking at a band: A band never owns anything — no group, no availability records of its own. It’s a tag on the parent faction’s list, nothing more. And two, only quality bands feed the RAT; a sub-command narrows what you’re looking at, it doesn’t change the odds. Future changes might see sub-commands feed their own RAT.
Member states, which are both
The interim hybrid is the member-state. It’s basically the fun case where a faction is sometimes independent and sometimes not — the Oriente Protectorate, the Regulan Fiefs, and others like them in the Free Worlds League are good examples.
A member state is a faction while it’s independent, and a band once it’s absorbed. One real-BattleTech faction, two stored representations, linked to each other. During its independence window it’s a normal faction with its own page. Outside that window it becomes a formation band on whoever absorbed it, and its units appear as part of that parent’s list.
This mirrors what actually happened rather than forcing a choice between “separate faction forever” or “never existed.”
Nothing is marked extinct
There’s no “extinct” flag anywhere in the MUL, and no Extinct faction. A faction appears in an era exactly when it has published availability records for that era, and it stops appearing when it doesn’t.
The Star League Defence Force isn’t flagged as gone after 2780. It simply has nothing to field, so it isn’t there. Faction lists are a consequence of the data and how we’re representing it rather than a separate thing to maintain — which means they can’t drift out of sync. It also means that perfect representation of the BattleTech universe, with its messy dates, and events that never land cleanly in an era, won’t align — and that’s ok. The MUL distills a growing catalog of 8700+ units into digestible chunks for new, current, and returning players alike. And remember, “whatever works for your game and your table,” trumps any data the MUL provides.
Record sheets, missing units, and mistakes — they happen
For those with a long memory, they’ll know the original MUL froze changes to the catalog in 2024 and only now has that freeze been lifted. There’s a massive backlog of units just sitting in draft that account for two real-world years of published material, along with the accrued errata, and availability tweaks that naturally occur over a 24 month period.
The same freeze was also applied to the majority of the record sheet library — which is currently available on the MUL — but slated for replacement as the new styles settle. ‘Mechs are the priority — to match the new Core Rulebook — with the other unit types to follow.
A lot of data moved around during the rebuild and some mistakes were bound to happen, most have been identified and already corrected, but something always slips through the cracks. The MUL was always intended to be a living tool and that hasn’t changed, so expect a steady stream of corrections to happen indefinitely.
Why write this info
None of this is visible when you use the site, which is the point. But it’s why the era dropdown only offers eras that have something in them, why a faction’s grouping is right in every era rather than most of them, why a faction isn’t in an era even though it might dip a toe or two, why sub-commands can have their own equipment profile without becoming their own powers, and why nobody has to remember to mark a realm as defunct.
It’s also, mostly, the shape the original volunteers arrived at over a decade of corrections. We formalized it. We didn’t invent it.
Lastly, please take an opportunity to read the What’s new with the MUL to get a sense of what the new MUL does and some of the new additions coming, and the MUL’s Help page, which answers many common questions about the application, including search. We’ll continue to add/revise Help information as time allows.
Joshua “Knightmare” Perian