What problem are you trying to solve? If there is a PRD, this can be a sync’ed block from the PRD. Link to any other documents that are relevant for background or context.
What should be true after this project is implemented?
What is explicitly not in scope and why?
What changes are required to solve this problem and achieve the project goals?
What are the high level architectural changes?
Diagrams can be very helpful here.
What are the high level data model changes?
These should include any database schema changes, or any changes to structured fields, e.g. an existing JSON column.
What are the main changes to the UI?
What risks might be introduced by this set of changes? Consider running a pre-mortem to raise risks. Be sure to capture mitigating these risks in the Implementation and Rollout Plans.