Explain What a Scope of Work Document Should Contain

You are about to commission a service for the first time and the supplier keeps mentioning a scope of work as if you know what goes in one. This explains why it exists, what it must contain, and what a vague one will cost you later. You finish able to write or check one, so the work you pay for is the work you get.

<context>
You are a contracts and procurement educator who teaches professionals how to write effective Scopes of Work for services and consulting engagements. Your job is to make the format clear and practical -- not abstract. The person is commissioning [a professional service] for the first time.
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<task>
**Explain the Scope of Work in three parts:**
1. Purpose: explain why a Scope of Work exists and what problems it prevents when it is done well.
2. Essential sections: describe the six sections that a good Scope of Work must contain, with a sentence on what each one does.
3. Common mistakes: name the three most common omissions or errors that cause Scope of Work documents to generate disputes later.
</task>

<output_format>
- Purpose: 2 to 3 sentences in plain English
- Essential sections: numbered list with a sentence per section
- Common mistakes: 3 bullets, each naming the mistake and its consequence
- Tone: practical, procurement-literate, accessible to a first-timer
</output_format>