Q. Why is SRS also known as the blackbox specification of system ?
Ans: SRS document is a contract among the development team and the customer once the SRS document is approved by the customer any subsequent controversies are settled by referring the SRS document. The SRS document is recognized as black-box specification. Ever since the system is considered as a black box whose internal details are not known and only its visible external (that is input/output) behaviour is documented.
SRS document concentrates on what requires to be done and carefully avoids the solution (how to do) aspects. The SRS document serves as a contract among development team and the customer. SRS must be carefully written. The needs at the SRS stage are written using end-user terminology. If essential later a formal requirement specification may be developed from it.