The requirements for the system are the description of the services provided by the system and its operational constraints.
It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification.
This is inevitable as requirements may serve a dual function
o May be the basis for a bid for a contract - therefore must be open to interpretation;
o May be the basis for the contract itself - therefore must be defined in detail;
Both these statements may be called requirements.