A Functional Requirement Specification or Functional Specification Document describes a Key document for the project created by the BA. The FSD outlines the intended behaviour of a system, including data, operations, input, output, and the properties of the system.
In a BRD the requirements are high level, but in an FRS/FSD, they are written in many more details to capture every aspect of a requirement. Thus a functional specification document becomes a more technical, accurate, and descriptive requirement document. Owing to their technical nature, FRS/FSD are equally used by developers, testers, and the business stakeholders of a project.