When a business analyst works on a team, the understanding of scope is very important. It is one of the key understandings that the project manager needs to deal with.
Part of the work a business analyst does is determine how they are going to determine the requirements and how those requirements fit with the scope.
Many times, it is when doing the initial analysis that the scope is determined. In the same way, a business analyst designs a solution for the client, they also need to design a solution for the project manager.
Determining how to best manage the requirements so that the project manager can track progress and remaining work is an important part of being a business analyst.
Sometimes this process is easy, the business analyst elicits the requirements and gains a solid understanding of the scope.
Other times, the business analyst has to work in levels. Get a high-level understanding of the whole system/change and then drill down into each one. This approach will still allow the project manager to put a plan in place that encapsulates the scope.