In this category there are countries, cities and areas, linked together in a hierarchy following that order.

Thus a country may contain several cities and a city can contain multiple zones.

Zones are typically reserved for major cities which justifies a better geographical distribution (see, for example Rome, London or Paris).
Is not usually a problem mapping the countries, but the mapping of cities and areas could be problematic because there are different “views” often implemented in various systems of booking online: if it usually is a trivial solution to integrate a system that does not provide the division in areas of cities as they tend to keep the whole place mapped under a single code (simply "ignoring" areas) can be a little more confusing a system that while not providing for “zones division” of a city wants to keep them as separate "cities" for different areas of interest (such as the Colosseum in Rome rather than the Eiffel Tower in Paris).

A meeting with the technical staff, however, always leads to the best possible solution to the problem.