UML has special representations for use cases and actors. A simple ellipse represents use cases; simple stick figures, actors. A use case diagram expresses the system's use cases in relation to the actors that invoke them. I prefer to construct these diagrams with an adornment that draws a boundary around the system, placing the use cases within the boundary and the actors outside (Figure 8-4). This notation is convenient when multiple systems are being jointly developed, and it can be clearly seen which use cases are part of which system. The complete collection of use cases, actors, and diagrams forms a use case model, which, like individual use cases, is just one part of the system's requirements specification.