Net systems form a hierarchy according to the functions S-elements play. At the bottom is EN-system in that a S-element is a condition; above it is P/T-system in that a S-element may hold more than one resource of the same kind, still above are predict/transition systems and color net system, in which a S-element may hold resources of different kind that play the same role. C-net system does not belong to this hierarchy, since it has variables as its consisting feature.