Writing for engineers, designers and stakeholders in one document, capturing non-functional requirements properly, recording decisions and their reasoning, and keeping a PRD current as the work proceeds.

For readers whose documents are now read by several audiences at once, each wanting something different from the same page.

You can write one document that serves several audiences, capture the requirements people forget to state, and keep a PRD accurate rather than abandoning it once the work begins.

Specifying behaviour at the edges, traceability from requirement to test, documenting integrations and data, requirements for work that is genuinely exploratory, and reviewing another authorx27s specification.

Written for people specifying complex or regulated work, where an ambiguity costs more than a rewrite would have.

By the end of this level you can specify edge cases before they are discovered in testing, trace a requirement through to how it is verified, and review someone elsex27s specification and find what is missing from it.

Deciding how much to specify for a given context, why documentation decays, standards people will follow without being policed, and the cost of writing down something that will change.

The deepest level, for readers who set documentation standards and must judge how much specification is the right amount.

The level leaves you able to set a documentation standard proportionate to the risk, recognise when a process has begun producing documents nobody reads, and decide deliberately what to leave unspecified.