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Functional Specification Document: What It Is and How to Write One?

Functional Specification Document

Every software project starts with clear goals, but things can fall apart fast when there’s no shared plan. Furthermore, projects often run into delays and confusion if the requirements are not documented properly.

The simple solution to this challenge is a functional specification document (FSD), which outlines what the system should do. You can think of it as a checklist to avoid confusion later.

Now, let’s understand more about the functional requirements document, its importance and core components, and how it is different from other documents, as well as how to write one.

What is a Functional Specification Document, and Who Uses it?

A functional specification document (FSD) contains information about product scope, functional requirements, input and output format, use cases, product overview, and associated risks. It acts as a blueprint for the software.

The simple goal of the FSD is to clearly define what the system should do and how it should behave in different scenarios from the end-user’s perspective.

Generally, multiple team members, such as Business Analysts, Project Managers, Product Owners, Senior Developers, etc., put collaborative efforts into preparing the FSD.

Moreover, FSD is used by multiple team members. For example:

  • Developers use it to understand what features they need to build.
  • Testers use it to create test cases and check if the system works as expected.
  • Designers refer to it to plan user flows and screen behavior.
  • Stakeholders and clients use it to review and approve the scope before development begins.

In short, we can say that FSD is the base for design, development, and testing teams.

Importance of a Functional Specification Document

According to the Reddit user, it is very important to develop a function specification document to ensure you have built the right solution. Another Reddit user considers FSD as a critical piece of design documentation in most cases.

According to our experience, here are a few reasons why FSD is important:

  • Defines a clear scope: The FSD clearly defines key features of the product. So, in the later stages, teams don’t make any assumptions about which features to include or not.
  • Ensures team alignment: FSD contains information about the product that is being developed. So, developers, testers, stakeholders, etc., equally understand what needs to be developed and remain on the same page.
  • Early gap detection: It helps teams to find gaps in requirements early. This reduces the chances of rework and saves both time and money for organizations.
  • Client Sign-Off: Makes it easier to get approval before development starts, avoiding back-and-forth later.
  • Better Testing: Gives testers a solid base to write test cases and check if each feature works as expected.

With FSD, every team member can understand their responsibility properly and avoid scope creep, which increases the overall efficiency of the team.

Components of a Functional Specification Document

The FSD can contain multiple components and sections, which may vary according to the industry or project. However, we have covered a few commonly used components below:

  • Project overview and scope: The project overview defines what problem we are solving and what product we are going to build. The project scope defines what to cover and what not to cover. So, teams don’t make any kind of assumptions.
  • Stakeholders: It covers who will be involved in the project and their responsibilities. For example, developers, testers, product owners, business analysts, project managers, etc.
  • User roles: This section defines who will be using the product. By keeping end-users in mind, teams build products that align with users.
  • Functional requirements: This is the core section of the FDS. It defines each functional requirement clearly and gives proper direction to developers for building the right product.
  • Use cases & user stories: It outlines how users will interact with the system.
  • System configuration: This section defines the steps required to configure the product. For example, the steps of account creation or login processes.
  • Approvals: Before starting development, it is very important to get approvals from key stakeholders. This section keeps track of features and decisions that have already been approved.
  • Risks and assumptions: It includes any risks associated with the project, including project delay, over budget, or technical requirements delivery.

BRD vs. FSD vs. SRS: Key Differences Explained

Point
BRD
FSD
SRS
Main Focus
Business goals and user needs
System features and user behavior
Detailed functional + technical needs
Audience
Stakeholders, clients, product team
Dev team, QA, UI/UX, project team
Dev team, testers, architects
Prepared By
Business Analyst or Product Owner
Business Analyst, Senior Developer, or Product Manager
Business Analyst or Tech Lead
Covers
What the business wants to achieve
What the system should do
How the system should work (in detail)
Level of Detail
High-level
Mid-level
Low-level, detailed and structured
Technical Content
None
Minimal
Technical and precise
Used For
Planning and stakeholder approval
Functional clarity during build
Final reference for development and testing
Document Style
More descriptive and broad
Actionable and use-case driven
Structured, often with standards and models

Related: Complete Guide to Writing Software Requirements Specification (SRS) Documents Like a Pro

Common Challenges in Writing FSDs

At Modern Requirements, every week we meet with multiple teams, and we observe that many teams regularly face the challenges below while creating and managing FSDs:

  • Hard to manage scattered documents: Teams use Google Docs and Microsoft Word for managing documents. When the number of documents grows and needs to be shared with multiple team members, it becomes challenging.
  • No link between requirements and work items: Teams write the FSD in Word or Excel, but the dev team works in project management tools, such as Azure DevOps. There’s no direct connection between what’s written and what gets built.
  • Difficulty in managing changes across teams: When multiple team members work on the same documents, it becomes challenging to track the document version history, and teams struggle to determine who has made which changes. It is a big risk during audits.
  • Back-and-forth during reviews: Teams generally make changes and then send documents via email for approval. With this, they need to manage scattered emails to collect feedback in notes, which is a very complex workflow.

To solve these challenges, you need a tool that allows you to create and manage documents, link requirements to documents, and manage reviews and changes. In the next section, let’s look at how Modern Requirements4DevOps can help with that.

How Modern Requirements4DevOps Helps in Creating FSDs

Modern Requirements4DevOps is a requirements management solution that directly works within Azure DevOps. Here is how it can simplify the process of managing FSDs:

  • Smart Docs: It allows you to create different types of documents directly within Azure DevOps. You can drag-and-drop functional requirements and add them to the document. So, whenever any requirement changes, the document is also updated.
  • Document management system: With this, teams can manage all documents within Azure DevOps.
  • Copilot4DevOps to write document: Copilot4DevOps is an AI assistant that comes with Modern Requirements4DevOps. It allows teams to use functional requirements as a reference and generate FSDs in seconds.
  • Review management: It helps teams to collaboratively review documents directly within Azure DevOps. So, feedback remains in one place.
  • Version control: It allows you to track the version history of the document, and if required, you can compare multiple versions.

This way, by choosing the right tool, you can simplify the process of FSD creation.

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