An amount of money. With regard to precision, see [Decimal Precision](datatypes.html#precision)
There SHALL be a code if there is a value and it SHALL be an expression of currency. If system is present, it SHALL be ISO 4217 (system = "urn:iso:std:iso:4217" - currency).
XPath:
(f:code or not(f:value)) and (not(exists(f:system)) or f:system/@value='urn:iso:std:iso:4217')