The objects used here are anonymous belonging to the classes shown. Sequence of messages are passed between the objects one after another. The messages are synchronous so that returns follow only after completion of the actions. The labels used show the order of events but are not necessary as the progress of time (top to bottom) ensures this order. This sequence diagram shows the scenarios for the particular use case. It is a generic diagram and include two alternative scenarios corresponding to printer being free and busy. It describes a dynamic situation and is an interaction diagram. It helps to specify the corresponding system functionality. Comments may be included on the left column of the diagram.