Types of Product Specifications

Functional specifications live in the middle layer of a 3-part hierarchy that every product team works within:

• Business or user requirement. It describes what users experience: "Users see a final cost reflecting the base price, any discounts, and promo codes applied." This belongs in a PRD or user story.
• Functional specification. It translates that requirement into how the product must behave: "A price calculation module must accept a base price, discount rules, and an optional promo code, and return a validated final price, even when no user interface exists for the calculation." This defines product behavior without prescribing implementation.
• Technical specification, which belongs to engineering. Architecture decisions, data models, and framework choices live here. A PM who writes at this level is overstepping.

The line between layers two and three is the hardest to hold. If your requirement describes what the product does, it belongs in the functional spec. If it describes how the code achieves it, hand it to engineering.^[1]

Technical specifications explain how a product will achieve the behaviors described in the functional spec. Using the same fitness app example, if the functional spec says “the app must display workout progress in real time,” the technical specification would define how this happens: which sensors capture data, which API updates the progress bar, what refresh rate ensures accuracy, and what database stores results.

These details form the engineering blueprint. They outline architecture, integrations, frameworks, and constraints. For instance, “the app must sync data every five seconds using Bluetooth Low Energy” or “the backend must support up to 10,000 concurrent users.” Clear technical specifications align teams on feasibility, performance, and security before development begins.

By separating user expectations from implementation details, teams can validate both product vision and system reliability without mixing goals.^[2]

Design specifications describe how a product should look and behave in detail, ensuring designers and developers share the same understanding of every element. They translate visual concepts into measurable design properties, bridging creativity and implementation.

A design specification usually combines two elements: a design file (often created in tools like Figma) and a development issue that provides additional context. The design file defines colors, typography, grid systems, and components, while the development issue clarifies goals, functionality, and any constraints. For example, a design spec for a mobile checkout flow might include button states, spacing between fields, animation speed, and accessibility notes like focus order or alt text for icons.

By standardizing these details, design specifications prevent inconsistencies and misinterpretation. They also help developers anticipate edge cases, like how a layout behaves on smaller screens or what happens when error messages appear. Good specs save time by making design updates visible and traceable across tools and teams.^[3]

Performance specifications define how well a product or system must operate under specific conditions. Unlike functional or design specs, which describe what something does or how it looks, performance specs describe how effectively it does it.

They include measurable benchmarks such as load time, speed, uptime, responsiveness, or energy efficiency. For example, a streaming app specification may require “video playback to start within two seconds on a 4G connection” or “the system must support 1,000 concurrent users without exceeding 70% CPU usage.” These metrics provide clear criteria for testing, making it easier to identify performance issues early.

Strong performance specifications protect both user experience and business goals. They ensure that products meet expectations across devices and environments, not just in ideal conditions. By setting quantifiable standards before launch, teams avoid vague statements like “fast” or “responsive,” which are open to interpretation.

Compliance and safety specifications ensure that a product meets legal, ethical, and technical standards before it reaches users. These documents outline the rules a product must follow to be approved, distributed, or used safely within its market. They can include accessibility requirements, data protection laws, environmental standards, or security certifications that apply to both digital and physical products.

For example, a medical app must comply with data privacy regulations that control how patient information is stored and shared. A connected device may require electrical safety or wireless certification before entering certain regions. By documenting these requirements early, teams prevent costly redesigns and delays caused by failing to meet compliance standards later in development.

These specifications also act as safeguards for users and the company. They reduce legal risks, protect user trust, and help maintain consistency across different markets and versions of a product. Clear compliance documentation ensures that every feature is both functional and responsible.^[4]

Not all specifications are written for the same audience. Internal specifications are created for team members within the organization, such as product managers, engineers, designers, and testers, who need detailed guidance to build and maintain the product. These documents emphasize functionality, architecture, and workflows that shape how the product operates.

When preparing internal specs, supporting materials often matter as much as the document itself. Flows that map what needs to happen at each step help engineering see the full picture. Designers can document all the cases the interface needs to support. Architecture can detail the technical foundation on which the product will sit. Together, these artifacts turn a spec from a static document into a shared working reference.

External specifications are shared with clients, partners, or regulators. They focus on outcomes and compliance rather than internal processes. Depending on the level of detail required, a short prototype or a product walkthrough video can communicate intended behavior more clearly than written requirements alone. Summarizing the goals being pursued or the main use cases being covered keeps external docs focused on what the other party actually needs to evaluate or approve.

Keeping the two types distinct helps manage expectations and confidentiality. Internal documents can include proprietary methods, while external ones communicate commitments that are safe to disclose.

Each type of specification serves a different group within the product team, helping everyone work toward the same outcome from their own perspective. But knowing who reads the document is only half the job. The format and depth need to match how that team actually works.

For internal technical teams, the medium matters as much as the content:

• Frontend engineers working directly on top of designs are often best served by the design files themselves, with specs that annotate interactions and states.
• Backend engineers, who rarely work from visual references, typically need workflow diagrams that map data flows, system triggers, and edge cases.

Using the wrong format for the audience creates friction even when the information is technically correct.

The same principle applies across the broader organization:

• Operations and support teams need to understand how things work in detail, including limitations and corner cases, because they are the ones who handle what happens when the product behaves unexpectedly.
• Sales and business teams rarely need that depth. A high-level definition of what the product does and what problems it solves is usually enough for their purposes.

Specifications do not exist in isolation. Each type connects to others at different stages of the product lifecycle, creating a continuous thread from idea to launch. Recognizing how they evolve together helps teams maintain context and prevent knowledge gaps.

Early in development, requirements or functional specifications describe what needs to be built and why. As the concept solidifies, technical and design specifications translate those ideas into detailed plans for implementation. During testing and release, performance and compliance specifications verify that the product works reliably and meets regulatory or quality standards. After launch, these documents often become references for updates, audits, or future versions.

Things change. Business priorities shift, engineers uncover constraints mid-build, and new information about users arrives at inconvenient moments. When that happens, updating the documentation is only part of the job. Teams may have already broken the spec down into stories or translated it into slide decks for business stakeholders. Those people need to hear about the change directly, not discover it the next time they open the document.

The more requirements are anticipated before development starts, the lower the risk during the build. If there is already a clear idea of how the rollout will be handled, that belongs in the documentation too. Sharing it early gives every team the context they need to plan their own work around it.