Although this document is usually described in the procurement knowledge area, it can also be seen as a major part of control that is needed in the project. It will be discussed again in the procurement area, but here is an explanation of the SOW.
According to the us, the SOW is "a narrative description of products, services, or results to be supplied." The SOW is extremely important on projects that will use materials and items from outside vendors. The better the description in the SOW, the better the vendor will be at offering an item that fits the needs of the project.
The SOW should be done in sufficient detail so that all collateral services are also included in the description. For instance, if you are planning an IT project that involves outside vendors who will do the actual code development, you should explain the product of the development you expect as well as how you want the vendor to report the progress on this part of the project.
The SOW is often seen with a Request for Proposal. For most government projects, an RFP is sent out to a list of authorized bidders. But a Request for Proposal doesn't give detail on the specific items. That is in the SOW. This is a thin line for many projects, but in PMBOK, the SOW is a specific document that you can use to control the costs of the project and to give various vendors as much information as possible about the type of items you want to purchase for your product.