Which sources count
Primary sources, meaning the body that issued the document: a statute, a federal statistical series, a standards organisation, a code body, a government audit. An aggregator summarising one of those is not the source, and using it introduces an error nobody can see.
Every source cited on a page appears in the sources block at the foot of it, with the publisher, the document title, the edition where the document has one, and the date it was read. The access date is there because documents move and are revised, and a citation without one is a claim about an unspecified version.
Sources are stored as records rather than typed into sentences, and the number in the text is rendered from the record. That is a technical detail with an editorial consequence: a figure cannot drift away from its source, because they are the same object.
What happens when a document cannot be obtained
Frequently, in this subject. Model codes and consensus standards are sold rather than published, several government sites refuse automated access, and documents move without redirects.
The rule when that happens is that the claim is not made. A document that could not actually be read is not cited, however confident the citation would look, because a citation to an unread source reads exactly as well as one to a source that was read. Where a fact genuinely matters and cannot be verified, the page says what is not established and describes how a reader establishes it for their own case. That is less satisfying than a number and it has the advantage of not being wrong.
Why no page prints a code requirement
In the United States these requirements acquire legal force one jurisdiction at a time. States and local governments each decide which edition to adopt and what to change in it, so two counties sharing a boundary can enforce different rules. A required insulation value, ventilation rate, snow load or electrical provision is therefore a fact about one jurisdiction on one date.
So pages here explain what a quantity means, what governs it and what to ask, and they send the reader to the authority having jurisdiction for the value. Every page that touches a code carries a standing note saying so. It is the most common criticism this publication receives and it is not going to change.
Review and revision
Each sheet carries a revision date at its head, and that date moves when the content changes. It is a literal date stored with the page rather than a build timestamp, so it means what it says: somebody changed this page then.
Pages are checked mechanically on every build against a set of rules: that every cited source exists and carries a publisher and an access date, that every page reaches its word floor, that no two pages share a passage, that no page makes a claim from the prohibited list, and that pages touching gas, water, electricity or structure contain no instruction to perform work. A page that fails any of those does not ship.
Corrections
Errors of fact are corrected on the page, the correction is stated rather than made silently, and the revision date moves. Errors of judgement are harder to adjudicate, and the response is to say what changed the view rather than to redraft quietly.
If something here is wrong, the editorial desk is the place to say so, and a specific correction with a source attached will be acted on faster than a general objection. The rest of the publication is on the contents sheet.