A spatial interaction between location A and B can occur only if three fundamental conditions are met:
- Complementarity. If location A produces / generates something that location B requires, then an interaction is possible because a supply / demand relationship has been established between those two locations; they have become complementary. The same applies in the other direction (B to A), which creates a situation of reciprocity common in commuting or international trade.
- Intervening opportunity. If location C offers similar characteristics (namely complementarity) than location B and is also closer to location A, an interaction between A and B will not occur and will be replaced by an interaction between A and C. An intervening opportunity can also be partial as only a part of the interaction is captured.
- Transferability. Transport infrastructures (modes and terminals) must be present to support an interaction between A and B. Also, these infrastructures must have a capacity and availability which are compatible with the requirements of such an interaction.