Yahoo! GeoPlanet is not a geocoder, so addresses in requests are resolved to the smallest bounding Place, usually a town or a postal code. Also, WOEIDs are not assigned to individual house numbers or street names.
Yahoo! GeoPlanet delivers bounding boxes and centroids of named places. This information is not definitive, and we make no claims to be the authority on the center of a place, its best routing point, nor the approximation of its extents. We provide coordinates only to assist users in finding the place on a map, and zooming to the correct extents.
Postal codes are assigned by postal authorities within a country for the purpose of expediting mail delivery, and do not necessarily align with administrative areas or cities in that country. For example, in the US, a city may be served by multiple zip codes, and a zip code may serve multiple cities. Where a city comprises multiple postal codes, the postal codes are represented in Yahoo! GeoPlanet as children of the city. Where a postal code comprises one or more cities, the cities are represented as children of the postal code. (This is usually a rural context and the cities are generally small towns).
If Yahoo! GeoPlanet does not return a postal code element for a place, then that place is served by multiple postal codes. A request for the postal code children of the place will often return this information.
Yahoo! aims to capture the geography of the Earth as it is used by the world's people. To this end we are guided by various standards and sources of geographical information. For country codes and names, we rely on ISO 3166-1 but make no specific claim as to official designation or authority of disputed territories.
We appreciate that the subjective and personal nature of world geography ensures that there is no single authoritative hierarchy and we do not aim to impose one here. Rather, the GeoPlanet hierarchy is presented to facilitate geographic discovery, and ultimately assist in disambiguating identically named places, and resolving spatial appellations to a unique, open, and permanent identifier.
Yahoo! aims to improve GeoPlanet at every opportunity, and we could always use your help. Please post comments, questions, and bugs (outrageous!) to the GeoPlanet Forum.