The promise to a customer is made before the forward deployed engineer's accountability starts, in the deck and on the second call, by someone whose number is bookings. A Forbes piece this month put FDEs on the sales team before the signature, and the job data says none of them carry a quota. The gap between those two facts is where promises get made that nobody owns.
Part 5 of The FDE Transition closes it with a one-page handshake: three columns, sales owning the number, the FDE owning the scope, and both names on the page before the signature, plus the quota and credit rules that keep the FDE from becoming a second sales engineer. The handbook chapter on the line between an FDE and a sales engineer is where the accountability test comes from, and building the FDE team has the reporting line and comp rules the post restates.