Cahier des charges (Spec Document)

In a client-provider relationship, this document usually forms the basis for the quote and the contract. On a redesign project, for instance, it settles questions upfront, like compatibility with an existing system or security constraints tied to the client's industry, instead of surfacing them mid-development. It's especially useful for tenders, where several providers need to quote against the same baseline.

The most common mistake is trying to lock down everything in the spec, down to interface details, which boxes in the technical team and makes the document obsolete after the first round of user feedback. On the flip side, a spec that's too vague on business goals opens the door to different interpretations between client and provider. The right level of detail fixes expected outcomes and non-negotiable constraints, without dictating the technical solution.