PRD vs SRD: what each is, and when you need which

A PRD (Product Requirements Document) and an SRD (Software/System Requirements Document) sit at different altitudes of the same build. The PRD is product-facing — the problem, the users, the outcomes, the features. The SRD is engineering-facing — the specific system behaviors, inputs, outputs, and constraints that make those features real.

They're often conflated, and that's where trouble starts: a PRD stuffed with technical detail loses the “why,” and an SRD written like a PRD leaves engineers guessing at exact behavior.

	PRD	SRD
Answers	What to build and why	How the system must behave
Audience	Product, design, stakeholders	Engineers, QA
Contains	Problem, users, goals, features	System requirements, inputs/outputs, constraints, acceptance criteria
Altitude	Outcome-level	Behavior-level
Written by	PM / founder	PM with engineering, or a spec pass

Which do you need?

You need the PRD first — always. It's the reasoning the SRD depends on. On a small team, the SRD often collapses into the PRD's requirements section plus the user stories, sizing, and test cases. On a larger or regulated build, you keep them separate so engineering has an unambiguous behavioral spec.

The mistake isn't picking one — it's writing one and pretending it's the other.

How the two connect

The clean path is PRD → SRD: the product reasoning produces the system requirements, not the other way around. Cadenly's PRD workflow builds the product side (problem → users → vision → features), and its Spec workflow produces the engineering side — user stories, sizing, test cases, tech stack, and architecture — so the two stay connected instead of drifting into contradiction.