A software requirements specification describes the essential behaviour of a software product from a user's point of view.
The purpose of the SRS is to:
Provide a basis for developing the software design. The SRS is the most important document of reference in developing a design
Facilitate transfer. The SRS makes it easier to transfer the software product to new users or new machines. Customers thus find it easier to transfer the software to other parts of their organisation and suppliers find it easier to transfer it to new customers