Managing scope is one of the most critical aspects of managing projects. However, if you have not done a good job of defining scope, managing scope will be almost impossible. The purpose of defining scope is to clearly describe and gain agreement on the logical boundaries and deliverables of your project. The business requirements are gathered to provide more detail on the characteristics of the deliverables.
Defining scope means that you have defined the project boundaries and deliverables, and the product requirements. These should all be approved by your sponsor.
The project manager and project team must realize that there is nothing wrong with changing scope - as long as the change is managed. If you cannot accommodate change, the final solution may be less valuable than it should be, or it may, in fact, be unusable.
Every project should have a process in place to manage change effectively. The process should include identifying the change, determining the business value of the change, determining the impact on the project and then taking the resulting information to the project sponsor for their evaluation. The sponsor can determine if the change should be included. If it is included, then the sponsor should also understand the impact on the project, and allocate the additional budget and time needed to include the change.