The SOW should not reference any external documents as its basis. To get your project right the first time, follow these guidelines for writing an effective statement of work, or SOW, as it's affectionately called.
Spell out the testing and support you'll need from the vendor, she says. A SOW should specify, for instance, that the end-user requirements are due two months after the contract is signed -- wording that still gets the project moving forward while accommodating potential problems such as a delay in signing the contract.
The SOW should be written as a definitive statement.
The problem becomes if you give them options in SOW, is that it is very difficult to determine which option they selected. This is not a best practice.
Also if you are audited, the auditors will look to ensure that all signatures are obtained including the internal agency signatures.