As a rule, we use roman numerals in the name of a compound when a metal cation forms two ions with different states of oxidation.
For instance, Cu can form two different ions – Cu+ and Cu 2+. Combining with chlorine, it will form CuCl and CuCl2 respectively.
So, the first compound, formed from copper 1+ ions will be called copper(I) chloride.
The second compound, formed from copper 2+ ions will be called copper(II) chloride.