Write the product PRD/spec only after the product opportunity brief is aligned and is a “go”.  

Here is my Product PRD / Spec Template. 

The product opportunity brief describes the problem statement, success criteria, why now, principles, product concept, and how to win. The PRD / spec goes further into the details and describes the prioritized use cases, the product / feature to build, and the release plan. It links to key artifacts such as the pre-release notes & FAQ, which the PM writes in parallel with the PRD. 

Is it ok to write both the opportunity brief and PRD in one document? Yes, though it’s important to align on the opportunity brief before doing too much work in detailing out the PRD. 

When to start the pre-release notes & FAQ? The PM starts it soon after the opportunity brief is a “go”; writing it right before release is too late. So, this and the PRD are written in parallel. 

PS: Typically I write about building products. I write to pay it forward and to clarify my thinking.