The currency of the discount. Use standard formats: [ISO 4217 currency format](http://en.wikipedia.org/wiki/ISO_4217), e.g. "USD"; [Ticker symbol](https://en.wikipedia.org/wiki/List_of_cryptocurrencies) for cryptocurrencies, e.g. "BTC"; well known names for [Local Exchange Trading Systems](https://en.wikipedia.org/wiki/Local_exchange_trading_system) (LETS) and other currency types, e.g. "Ithaca HOUR".