We can catagorise settlements according to their size and shape. The result is a settlement hierarchy.
As you move up the settlement hierarchy the size of the settlement increases, as does the population and the range of services available. Smaller settlements tend to provide only low order services such as a post office and newsagents. Whereas, larger settlements have more high order services such as leisure centers and chain stores. As the result of this the larger the settlement, the greater the range of services and therefore the market area or sphere of influence. This is the market area that a settlement services (the distance people will travel to use services).
High order services usually have a high threshold. This means they need a higher number of people to use the service in order to remain profitable. This means high order service such as department stores need a greater number of customers than a low order service such as a newsagent. This is why there are so few department stores in villages!