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In a military occupation, the occupying country takes control of
the occupied territory's governmental apparatus, typically including
the postal service. The occupier may either continue to allow the
occupied's stamps on mail, or issue new stamps of its own; the
occupier's own stamps cannot usually be used as-is, since they will be
in a different currency, and the range of values will not reflect
the occupied's postal rates.
Philatelically, occupations of country X by country Y are normally grouped with X rather than Y.
In StampData, each occupation is its own distinct type of issuer, with a type of "occupation". The occupation's main country is the original country being occupied, while the occupier has the property "occupier". Occupations are sorted to be listed after the occupied issuer.
In some cases, such as the Japanese occupation of China, an occupation may have sub-issuers, for instance Chinese provinces like Hopei and Hupeh.