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 — two artefacts, both produced when a tranche closes. The closed Milestone with its attached closed Issues, which is both the physical signal that close-out happened and the forge-derived record of what the tranche shipped. And the retrospective, posted to the standing lessons thread — the durable record of what stalled and what carries forward. (The hand-maintained product state record that once also crossed here is retired — anything still outstanding is an ordinary open Issue, closed when resolved.)
The hand-off is malformed when — either 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 closed Milestone 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.