It is a transportation supply and demand relationship that is often expressed over a geographical space. Spatial interactions usually include a variety of movements such as travel, migration, transmission of information, journeys to work or shopping, retailing activities, or freight distribution.
Edward Ullman, perhaps the leading transportation geographer of the twentieth century, more formally addressed interaction as complementarity (a deficit of a good or product in one place and a surplus in another), transferability (possibility of transport of the good or product at a cost that the market will bear), and lack of intervening opportunities (where a similar good or product that is not available at a closer distance).
The first factor necessary for interaction to take pace is complementarity. In order for trade to take place, there has to be a surplus of a desired product in one area and a shortage or demand for that same product in another area.
The greater the distance, between trip origin and trip destination, the less likelihood of a trip occurring and the lower the frequency of trips. An example of complementarity would be that you live in San Francisco, California and want to go to Disneyland for vacation, which is located in Anaheim near Los Angeles, California. In this example, the product is Disneyland, a destination theme park, where San Francisco has two regional theme parks, but no destination theme park.
The second factor necessary for interaction to take pace is transferability. In some cases, it is simply not feasible to transport certain goods (or people) a great distance because the transportation costs are too high in comparison to the price of the product.
In all other cases where the transportation costs are not out of line with price, we say that the product is transferable or that transferability exists.
Using our Disneyland trip example, we need to know how many people are going, and the amount of time we have to do the trip (both travel time and time at the destination). If only one person is traveling to Disneyland and they need to travel in the same day, then flying may be the most realistic option of transferability at approximately $250 round-trip; however, it is the most expensive option on a per person basis.
If a small number of people are traveling, and three days are available for the trip (two days for travel and one day at the park), then driving down in a personal car, a rental car or taking the train may be a realistic option. A car rental would be approximately $100 for a three day rental (with for to six people in the car) not including fuel, or approximately $120 round-trip per person taking the train (i.e., either Amtrak's Coast Starlight or the San Joaquin routes). If one is traveling with a large group of people (assuming 50 people or so), then it may make sense to charter a bus, which would cost approximately $2,500 or about $50 per person.
As one can see, transferability can be accomplished by one of several different modes of transportation depending on the number of people, distance, the average cost to transport each person, and the time available for travel.