A statement of work specifies what will be delivered, by when, and what counts as done. For an FDE it is not paperwork to be ignored: the acceptance criteria in it decide whether the customer pays, and they are frequently written by people who were not going to build the thing.
Worked example: an SOW clause saying “95% accuracy” with no definition of accuracy, no held-out set and no error-cost weighting is an unwinnable commitment; replacing it with “match or exceed the measured expert-agreement baseline, with under 2% high-cost errors” makes it testable. Gotcha: read it before signature, not at handover — an FDE inherits every promise made earlier in the sales cycle.