A Statement of Work is the document that defines what a vendor will deliver, by when, and for how much, on a specific project. It sits underneath the MSA and inherits the MSA's legal terms. The SOW is where the work gets specified. The MSA is where the rules get set.
A defensible SOW covers six items. Scope, written tightly enough that a project manager can recognize "done" without arguing. Deliverables, named and dated. Acceptance criteria, including what triggers customer acceptance and what happens if acceptance is delayed. Pricing, with milestone or deliverable triggers if it is not a fixed fee. Assumptions and dependencies, especially around customer-side resources. Change-control mechanism, because every project of any size has scope creep.
The recurring drafting trap is a vague scope clause. "Provide consulting services as mutually agreed" is not a scope; it is an open invoice. The vendor benefits in the short term and pays for it in the long term when disputes about what was actually in scope show up at renewal or termination. A tighter scope is better for both sides. The customer gets predictability. The vendor gets a legitimate basis for a change order when the work expands.
Acceptance is the second recurring trap. SOWs that say "deliverable accepted on customer satisfaction" hand the customer a unilateral non-acceptance veto. Acceptance criteria need to be objective: what test does the deliverable have to pass, how long does the customer have to test it, what happens if the customer does not respond. Done well, acceptance is a date-driven mechanism that closes out the project on a predictable timeline.