A normal coin series consists of subunits and sometimes also units. To explain: the currency of my country, the UK, is the pound. The pound is the prime unit of currency, and it is divided into subunits of 100 pence. The unit is the pound, and the subunit is the penny. When the UK adopted a decimal currency in 1971, the highest denomination of coin was the 50 pence. Nowadays, we have a circulating 2 pound coin - or 200 pence, if you prefer to see it that way. Over time, of course, inflation makes it more efficient to use high value coins than banknotes.
If inflation is relatively high, then over a period of years your subunits will become worthless. Your remaining coins will then be denominated in units, and the denominations become higher and higher as inflation progresses. For instance, in 1977 Angola adopted the kwanza, which was subdivided into 100 lwei. The lowest denomination of coin at the time however was the 50 lwei coin. In 1990 the New Kwanza was adopted, and the lowest denomination was now 50 kwanzas. Inflation had killed the lwei. However, in 1999 Angola redenominated the currency and introduced the second kwanza, which was now subdivided into 100 centimos instead of lwei. The lowest denomination of coin in 1999 was 10 centimos.