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 reciprocity common in commuting or international trade.
- Intervening opportunity. If location C offers similar characteristics (namely complementarity) to location B and is closer to location A, an interaction between A and B will not occur, as an interaction between A and C will replace it. 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 compatible with the requirements of such an interaction.