Canon comes from the history of the Romans and the Catholic church and the idea of rules and law to govern. It is literally a "need" for society to function. In this case BattleTech Canon is needed to play a game of BattleTech.
The BattleTech canon is quite interesting if you look at it as a liturgical process. By this I mean I am equating being a fan of the game as "worship", and that "service" is following the rules of the game. There is no implication of religious idolatry here, just a functional behavioural comparison that sees both as being a similar process.
Liturgy derives from the ancient Greek concept of service to the state, and carried with it a financial burden taken on by the citizen. Using this we would see that buying BattleTech products as the financial burden undertaken so as to be able to follow the rules of the game; the "service".
As you can tell this post was driven by my previous post about citizenship brings service.
Rules are generally written down in books, and law is the literal enforcement of the rules. So, when we play a game of BattleTech (or for that matter any other game with a set of rules) we are engaged in a lawful act (thereby being good citizens). No wonder people get involved with arguing the finer points of the rules, because otherwise they would be seen as acting unlawfully (this is probably worth a post all of its own, and again I find myself inspired by my recent experience as a juror).
The first canon, now redacted, were those rules found in the BattleDroids game from FASA. Unfortunately, I no longer have a copy of this book, because as soon at the second edition of the game came out I moved to using those rules, because they were expanded and had clarified, or changed how things happened. For instance the sequence of actions within a turn were changed, and the construction rules for making battlemechs were also changed, as just two examples.
From this canon came the first Technical Read Out, or TRO as I will now refer to these works.
The famous and much loved TRO:3025. I will now suggest that a TRO is a catalogue of legal things that the rules allow you to use. This is a bit like "case law" under the British legal system. Case law is known as common law from precedent, which means quite simply if something has been agreed on before that it is only fair to apply the same ruling when it happens again. So if a battlemech designed under the rules appears in a TRO it is part of the canon of BattleTech.
So by extension all books that are based on the game canon, become canonical, which means it is the normal standard one plays by. However, just to remind you, I am not talking about BattleTech being a religion whereby errors and omissions become heresy, but a game where the terms and conditions are that errors and omissions will be corrected in later errata, which is more like British case law that I mentioned earlier. To be continued...