THE SHORT ANSWER
The operational knowledge: how your process really runs, the exceptions and unwritten thresholds, which the engineer applies at the next customer. Milos Mandic's October 20, 2026 FDE Hub post names it plainly: one former head of FDE told him around 70% of FDE output should flow back into product, and his own sixth client got the value of the first five without anyone signing anything that said so. The contract covers code, prompts, configurations, and business rules. It cannot cover the learning, and asking the engineer produces a paragraph and no record. The record exists: the handback packet an FDE owes product, with the delta in configuration terms, the customer's eval rows, the reversals, and the count of other customers with the same problem, due at two weeks, go-live, and day 60. The buyer should receive a copy at the same three moments. The count column says how much of your process is becoming the vendor's roadmap.
Do not ask what left. Get a copy of the packet, at the same three moments the vendor's product team gets it.
The rewrite, the one place I differ with Mandic's framing, and the four acceptance criteria are in What Leaves With the Engineer: Get a Copy of the Packet. The packet itself, the delta, the rows, the reversals, and the count, with its three due dates, is defined in Absorb Pain, Excrete Product: The FDE Handback. The keys line, the operator who holds the pen after the engineers leave, comes from Month Two: The FDE Leaves, the Workflow Has to Stay.