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A currency is the system of money used in a country or other
As fiscal instruments, all stamps have at least one currency implicit in the denomination, even if the valuation is not written on the stamp, or is variable in some way (nondenominated stamps, forever stamps, etc). A small number of stamps are dual currency; examples include early Canada, early New Hebrides, and Rhodesia.
The definition of a currency as a series of numbers and currency unit abbreviations that are all of equal value to each other. So for instance the traditional pre-decimal pence/shillings/pounds system is defined as "240d=20sh=1lb". If one wanted to allow both "d" and "p" for the pence, the definition would be "240p=240d=20sh=1lb".
The numbers are used to calculate the "canonical value" (need a better term) which is suitable for sorting stamps by increasing value, irrespective of how the stamp's denomination is written.
A currency may be either unique to an issuer, or shared by many issuers. A currency is only legitimately shared if the currency is really the same system, and not just written the same; both the United States dollar and East Caribbean dollar have currencies defined as "100c=1$", but they are different currencies.
(As an interim workaround, if the currency description says "new", it will be added to currency selection popups. This is so newly-created currencies can be applied to stamps, otherwise the popup only shows currencies already in use.)