A Functional Specification document is a blueprint for like how we want a particular project or application to look and work. It also details what the finished product will do and how a user will interact with it. The Functional Specification is in essence a contract between the business customer and the IT project team, describing from a technical view what the customer expects.
Business Analyst translate the business requirements into system functionality in technical terms. BA, personally, worked with tech lead to come up with the high level architecture. This system architecture can be broken down in functional modules based on the requirements. These functional modules are component of the system, which can be clearly distinguished from the other system. In between, initial recommendation of the technology will also help in designing the functional specification document. I documented the functional module behavior in use case diagrams and use case scenarios.
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