The role is easiest to understand by what it is accountable for. A sales engineer is accountable up to the signature. A consultant is accountable for the recommendation. Professional services is accountable for the spec. The forward deployed engineer is accountable for the outcome, in production, on the customer's real data, after everyone else has gone home.
That is why the role is exploding now. Code got cheap, so the scarce work moved from writing software to making it produce a result inside a specific business. And AI vendors started pricing on outcomes, which requires someone to guarantee the outcome. Both point at the same seat.
The full treatment is a seven-chapter wave of the handbook, starting with the thesis chapter linked above. If you only read one, read the chapter on where the FDE starts and the sales engineer stops, because that is the confusion that costs the most.