An operation note (note: Operation) that names an API (note: API) with an imported contract (note: Contract), in its api attribute (note: Attribute) or through a markdown link (13 passages, no note), and that none of the three rungs of the operation matching attaches to an operation of that contract. One of two things is true: the operation disappeared from the contract and the note (note: Note) describes something that no longer exists, or the note is ahead (3 passages, no note) of the contract and describes an operation the next version will declare. The build (note: Build) cannot tell which, so the finding (note: Finding) says both, and the author (note: Author) decides: retire the note or point it at the operation that replaced its own, or keep it with the operation_id to come until the contract catches up. A note that names no API is not reported: it is a candidate (note: Candidate) for every contract of its source (note: Source) and may describe an API without a contract. A note taken in an ambiguity is reported by the matching rule (4 passages, no note) instead.
The gap (8 passages, no note) is measured in the other direction on the page of the API: the contract viewer lists every operation of the contract (3 passages, no note) and flags (5 passages, no note) the ones no note describes yet, with their count. Together, the finding and the flag give the documentation (12 passages, no note) debt of an API: what the notes say that the contract does not, and what the contract declares that the notes do not.
Check (note: Check) W-OPERATION-UNMATCHED, severity (note: Severity) warning by default, documented with a before and after example in the check pages. The severity can be overridden per project (note: Project) or per repository (note: Knowledge repository).