What is fact lineage?
Fact lineage is the record connecting a number to the exact document it came from. A figure with lineage can be resolved back to its source filing, the period it covers, and the moment it became public — so a reader can check it rather than trust it.
The idea predates AI and matters more because of it. When a person copies a figure into a memo, the chain of custody lives in their notes and their memory. When a language model produces the same figure, there may be no chain at all: models generate text that resembles the training data, and a plausible number is indistinguishable from a filed one once it is sitting in a sentence.
Lineage makes the difference visible. If every figure an assistant emits carries an identifier that resolves to a filing, then a number without one is conspicuous, and the question "where did this come from" has an answer that does not depend on the model being honest about itself.
This is also what makes AI output defensible after the fact. A research file that records which figures were consulted, from which filings, is the artifact you hand a client, an investment committee, or an examiner — and it is worth substantially more than an assurance that the model was careful.