A functional requirements document (frd)—also known as a functional requirements specification, functional specification, or functional specification document—is a more. Writing a functional requirements document (frd) is an essential step in the software development process. Are you wondering what a functional requirements document or what an frd is? Documenting the functional requirements provides several important benefits to the organisation and to the process of developing a solution. [provide a list of contractual or task level assumptions and/or constraints that are preconditions to preparation of the frd.