A currency is assessed for exchangeability at a measurement date and for a particular purpose (such as realising or settling items, paying dividends, or translating a foreign operation's results). It is exchangeable when the entity can obtain the other currency, directly or through a third currency, within a period allowing for ordinary administrative delay, using only markets or mechanisms in which an exchange would create enforceable rights and obligations. What counts is the entity's ability, not its intention or decision. Different rates may exist for different uses, so the purpose matters, and exchangeability may hold in one direction only. If the entity could get only an insignificant quantity compared with the total it needs for the purpose, the currency is not exchangeable.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.