Functional versus technical design specifications
Key Takeaway
The FSD describes what the solution does from the user's point of view, but only where behavior deviates from the reference application's standard functionality. The TSD describes how the solution is built, down to data mapping and the system integration plan.
The functional specification document (FSD) and the technical specification document (TSD) are sister documents that turn business requirements into something a team can build. The FSD explains what the solution does from the user's point of view. The TSD explains how the solution does it, with the precise details developers need to build it. After both documents are complete, the team compares them to the earlier scoping documents to find changes in scope.
What each document answers
The FSD answers what the solution does. It describes behavior from the user's perspective, not how that behavior is built.
The TSD answers how the solution does what it does. The TSD holds the precise technical details for building the functionality that the FSD describes.
The two documents serve different readers. The development team builds from the TSD. Later, the technical support team uses the TSD to understand implementation details. Aim to include as much detail as possible so the implementation team knows exactly what to do.
A manufacturer wants dealers to see each machine's service history. The FSD says what the dealer experiences: on a machine's page, the dealer sees the five most recent service visits, newest first. The TSD says how the team builds that page. The TSD covers where service visits are stored in the data model, how visit fields from the maintenance system map to those fields, and when the maintenance system sends new visits.