Read First: Operation Gives the information available to the user of an operation after the operation is done. For example, if an operation fills an order, the order may be output. The information that is output by an operation is unrestricted in OOIE. Usually the output will include the objects that have their state changed by the operation, and which are refered to by the resulting event types (see resultsIn/isResultOf). Since the output variables are defined for an operation independently of which event diagrams use it, they should cover all information that might be needed in any event diagram.