Generally  in FRS and BRS ,the functionality of an 
application is described in terms of Usecases.
  what u said is right usecase describes the functional 
requirements  in terms of UML diagrams.
   From usecases we  derive abstract testcases and from 
that again we write low level testcases.

hi meenakshi,
what ever answer u have provided is not correct,as use case 
r the document which show the functinality in the form of 
UML diagrams and Explain the flow of data between the 
actors and process,where as BRS,FRS r documents which store 
the functionalities in readable form(English)

a test engineer refer BRS,FRS for writing the testcases and 
refer Usecase diagram to clearly understand the 
functionality(like the flow)

Na is absolutely right use cases are used to describe the 
interation of the actors with the system.and BRS & FRS are 
the documents that are written with the understanding  of 
Uses cases.