By default this setting is off and the catchment area of each station tile is 4.
With this setting enabled, the catchment area around a 'station' varies depending on what type of station it is. For example, train stations have a larger catchment area than bus stations and airports' catchment areas are bigger still. It even varies with the type of airport; Intercontinental Airports have the biggest.
With realistic catchment:
|Road Stop||Buses, Road vehicles||3|
|Airport||Aircraft||4—10 (but NewGRF-configurable)|