In spite of the important distinction between linear and nonlinear systems, many general properties of such systems depend only on the nature of the connections in the system. Thus a strongly connected system has paths from any compartment to any other so each compartment can influence any other. Another important distinction is outflow connectivity. An outflow connected system has paths from any compartment to some compartment that has flow out of the system. Consequently, if there is no inflow, all outflow connected systems eventually empty to the outside world. Finally, another important distinction is a trap. A trap is a subset of compartments which have no outflows. Consequently, material entering such a subset can never leave, it is trapped.