Transport supply is generally expressed by Aij; the transport supply between location i and j. Indirectly, it combines modal supply, the capacity of a mode to support traffic, and intermodal supply, the capacity to transship traffic from one mode to another. For instance, air transport supply between two cities must jointly consider the capacity of the airports as well as the capacity of air corridors. The supply of some transport modes, such as road transport, is more contingent upon the modal capacity, while the supply of other modes, such as maritime and air transport, depends on intermodal capacity.
Transport demand is represented by Tij; the transport demand between locations i and j. The potential transport demand is the amount of traffic if transport costs were negligible or if there were almost no constraints on mobility. The transport capacity would be unlimited. The realized transport demand, a subset of the potential transport demand, is the traffic that actually takes place, namely in the function of the transport costs between the origins and the destinations. Therefore, the realized demand is an outcome of the constraints imposed by the existing transport supply.