The unit for trading stock indices is an index. Each stock index will also have its own currency depending on its country. Below we see the currency associated with each of the major indices.
Stock Indices Futures
So if we are buying or selling a US index it will be priced in US Dollars. The FTSE will be priced in Pound Sterling. The CAC and DAX will be priced in Euro. And the NIKKEI will be priced in Japanese Yen.
So as we should know by now when we are buying and selling instruments using leverage we do not need the full amount of the position in our balance. Instead we only require a portion of the total value known as the margin requirement