How does a system-design document differ from a PRD?

How does a system-design document differ from a PRD?

A Product Requirements Document, or PRD, explains what a product should accomplish and who it serves. A system-design document explains how its software components will deliver that behavior. They answer different questions while describing the same product.

Separate product outcomes from implementation

A PRD might say that an organizer can invite a guest to selected wedding events and see whether the guest has responded. It should define the user journey, access expectations, and acceptance criteria.

The system-design document translates that requirement into responsibilities: where invitation records live, which server endpoint creates them, how delivery happens, and how a response updates the dashboard. It also covers failures such as a message provider being unavailable.

Connect requirements to technical decisions

Give important requirements stable identifiers and reference them from the technical design. For example, an invitation requirement could connect to the invitation module, a response endpoint, and a rule that guests can only view events included in their invitation.

This connection helps an AI assistant distinguish required behavior from optional implementation ideas. It also makes omissions visible: a feature without a technical owner needs further design, while a component without a product purpose may be unnecessary.

Keep both documents consistent

The PRD should avoid prescribing every database field unless the field represents a real product requirement. The architecture document should avoid quietly adding features because a library makes them possible.

Review them together before implementation and after meaningful scope changes. If the product begins supporting multiple organizers per wedding, update the user roles in the PRD and membership checks in the system design. Neither document needs to be frozen forever; both should remain understandable records of the current agreement.