FDS

Functional Design Specification. Defines “what” the system should do, rather than “how” it is technically built (which is for a Technical Specification Document).

In the context of software development, engineering, and IT project management, FDS (Functional Design Specification) and FSD (Functional Specification Document) are generally used interchangeably to describe the same type of document. Both documents outline the functional requirements of a system, detailing what the system must do, its features, user interactions, and expected behavior from an end-user perspective. 

FDS (Functional Design Specification): Sometimes implies a stronger focus on the design aspect, describing how a process or control system will operate. FSD (Functional Specification Document): Often focuses on defining the requirements for a specific function or enhancement, serving as a bridge between business needs and technical implementation.