Built elsewhere
When an item is already being built outside the engine, a planned route for it would build it twice. Now a person can say so: the route goes, no engine picks the item up, and it still lands from its PR.
A big item on the sled gets a route planned for it: the engine reads the code, splits the work into nodes, and leaves the plan in Needs you for a person's yes. That is the right default. It is wrong when someone is already building the item another way, by hand or in Claude Desktop, say, before the engine got to it.
Until now there was no good answer to that plan. Approving it would start a second build of the same work. Running it as one node would do the same. The only safe move was to leave it sitting in Needs you and step around it until the item landed.
What happens now
Mark it Built elsewhere. The planned route is discarded and the entry leaves Needs you. No engine plans the item again, runs it, or takes it, not even as a single node. The item's thread gets one line saying who marked it and why, with your note when you leave one, so anyone who opens the item knows where the work is happening.
The item stays on your sled. When the PR you are building merges with its SLEDS-Source: line, the item lands as any item does, and whatever was waiting on it is ready to start.
Changed your mind? Run as one node, or drawing a route by hand, takes the mark back and puts the item in the engine's hands again.
Why it matters
A planned route is a question for you, and a question you cannot answer is noise. Built elsewhere is the honest answer for work that is already moving: it keeps the engine from duplicating it, keeps the item where you can see it, and keeps the record whole when the PR merges.
It is the first way to clear a Needs you entry without acting on it. A general dismiss for every kind of entry comes later.