The SOW is typically a discrete document, either a physical hardcopy or electronic. Electronic copies tend to be distributed in PDF format making them more official and difficult to modify.
Depending on the dollars involved, whether the project is internal to an organization or an understanding between corporate entities, the SOW will become more or less formal. On the high-end, the SOW may be a legal document that must be approved by attorneys, or a schedule of a Master Services Agreement.
In the case where the SOW is a legal document, it will likely degenerate in its operational value to project team members. The sheer inclusion of legalese will make much of the document unreadable to the IT participants, and attorneys will likely either not allow or dilute contents that are valuable to hands-on project management. In cases where corporate legal departments must have a legal SOW document, it might be best to have it refer to an “exhibit” document which can be used for the operational management of the project.
For smaller, simpler and/or well-defined projects, the SOW may be a self-contained document. For larger projects, it may be a primary entry point that references other documents or agreements. For software development projects, for instance, the SOW likely cannot quantify all customer requirements, either due to the volume or because they are to progressively elaborated. Thus there will be discrete requirements documentation. Also, depending on the particular methodology followed by the PM, the SOW may be a separate entity or part of a larger project plan.
For infrastructure projects and probably the majority of IT projects in general, the SOW will likely be the only document that is executed prior to work commencing.
Some teams, especially those without a dedicated project manager or involved in an internal project, may wish to avoid the administrative paperwork and get right to business. Because of prior successes, a good relationship with the customer, or for the sake of expediency, a team may feel a handshake, verbal agreement, or some vague meeting minutes might serve the purpose of a SOW.
Resist this urge!! Business conditions, personal agendas, subjective interpretation, and selective or defective memory will eventually manifest into a disagreement on scope, schedule or another variable and cause turmoil, if not outright project failure.