Functional currency
The currency of the company's primary economic environment — the one it earns and spends in; books are kept in it before translation into the presentation currency.
How to read the number
Explains why a weaker rouble does not always hand an exporter a paper gain: if the books are kept in the currency of its revenue, translation hits other items instead.
When the metric lies
It need not match the presentation currency and is not a free choice: it follows from the facts of the business, and changing it changes the whole series.