How do you know the magnitude of a project? How to derive project estimates? How do you know what to build? Or in testing terms, what should be the expected behavior of a software that you are testing? High-level requirements, Software Requirements Specification (SRS), Functional Specification Document (FSD), User Stories or Use cases – whatever you call it in your project – is essentially a requirements documents which forms the basis for software development & testing. A Software requirements specification (SRS) is a description of features and functionalities of a software system to be developed.