A diagram will contain a combination of events, results and processes (and optionally holds). Events can either generate objects or receive objects from the preceding process as indicated by the parent. It is legal to add object generators at any level of decomposition however there should always only be one event that receives objects from the preceding process via the parent.

Objects can be terminated at any level of decomposition but a single result is designated to pass objects onto the next process via the parent. The reason for this rule is that it would be impossible to represent the complex logic involved in having multiple results feeding the next process in the parent diagram (it also makes good modelling sense). The validation routines will check this rule prior to running the simulation.