The required reserve ratio is set by the central bank as a percentage of attracted funds and may vary by type of obligation — in particular, for deposits in national and foreign currency.
The instrument serves a dual function. It affects the volume of funds available to banks for lending and is therefore part of monetary policy. At the same time, reserves serve as an element of banking system stability.
An increase in the ratio withdraws liquidity and makes lending more expensive, a decrease releases funds. Compared to interest rates, this is a cruder instrument, so it is used less frequently and usually as a supplement to the main one.
The difference in ratios by currency has a direct practical consequence for depositors: higher requirements for foreign currency obligations make attracting foreign currency more expensive for banks, which is reflected in the rates offered. This is one of the reasons why the spread between domestic and foreign currency rates is not determined solely by exchange rate expectations.