Saeed,
Here is what the PMBOK Guide says:
Requirements Documentation: A description of how individual requirements meet the business need for the project
Project Scope Statement: The description of the project scope, major deliverables, assumptions and constraints
To simplify... one describes the requirement -- for example "we want to entertain our customers" -- and the other describes the scope of how we are going to achieve this requirement -- for example "we are going to hire The Rolling Stones to play music". As you can see, the requirement of entertaining someone could be achieved through different solutions/scope. We could show them a movie or put on a theater performance.
Therefore, if your customer says "it doesn't meet the requirements" then you compare this against the acceptance criteria defined in the requirements documentation, and if the customer says "this doesn't meet the scope definition" then you compare it against the acceptance criteria from the scope.
Until Next Time,
Cornelius Fichtner, PMP, CSM
President, OSP International LLC