This seam sits between the end of one tranche and the planning of the next. It exists because planning starts by reading the current state of a product, and a tranche that was never closed out leaves those records describing a product that no longer exists.
What crosses — three artefacts, all produced when a tranche closes. The archived tranche itself, which is the physical signal that close-out happened at all. The product's state record, brought up to date: what it is now working toward, which manual steps are still outstanding, and what the tranche just shipped. And the retrospective, posted to the standing lessons thread — the durable record of what stalled and what carries forward.
The hand-off is malformed when — any of the three is absent. Each has a matching check on the planning side, and the planning side stops rather than working around it: the archive is a fact to be confirmed, not an assumption to be made. "It was probably closed out" is not a passed check. The failure this was written against is real: tranches completed without close-out, and the next plans were then built on records describing the product as it had been before.
What it does not carry — a statement of what happens next. That is not archived, it is derived: the open work on the forge is the answer, read directly rather than maintained by hand in a file that would drift. Nor does it carry the power to move a task between tranches; that is a scoping decision, and it belongs to planning.
How it physically runs — the carriers are the archived tranche, the product's state record, and the retrospective comment. The ordering is normally close first, then plan. One exception: when a new plan absorbs an existing tranche's unstarted work, the move happens first and the close-out follows, because the close cannot proceed while that work is open and the plan is what empties it. Only unstarted work may move — anything with a branch or an open pull request is finished or dropped where it is, never relocated mid-flight — and every move leaves a note on the task, so a task that changed address can be told from one that vanished.