I often use two types of context diagrams:  one to document the work scope in the beginning of the project and another to document the product scope of the system to be developed.  The work scope context diagram looks at the business process as a single black box with its interactions with outside entities.  It acts as a communication tool that allows everyone invovled to agree on the work that is being looked at for the new system.  The product scope context diagram is more focused on the system to be developed.  It treats the system as a black box and shows the inputs and outputs to and from external entities.  Both are similar, but after doing some initial analysis, it may be determined that the product scope is smaller than the work scope.