Ask one question about any customer-facing technical role: when the customer's outcome does not show up, whose number goes red?
A sales engineer's number is bookings. Once the deal closes, the outcome belongs to someone else, and the incentive structure is honest about it. That is a real job, and the best SEs are extraordinary at it. A forward deployed engineer's number is the outcome. At Sierra that is literally the invoice, because the customer pays per resolved case.
Everything else people list as a difference is a consequence. FDEs are on site more because you cannot own an outcome from a demo environment. They write production code because the outcome lives in production. They stay after go-live because that is when the outcome is measured.
The handbook chapter above walks through all five neighboring titles with the same test, and the transition chapter covers what an SE has to change to make the move, which is shorter in skills and longer in habits than it looks.