What kind of finance numbers do you keep, that you need 15 digits of accuracy. Exchange rates are usually 4 digits. So where does the other 11 go and why do yo

What kind of finance numbers do you keep, that you need 15 digits of accuracy. Exchange rates are usually 4 digits. So where does the other 11 go and why do you need it? For example, this 0,00000000001 of a dollar?