To convert Arabic numeral (Western numeral i.e. everyday numbers) you have to separate number into thousands, hundreds, tens and units. With Conventional Roman numerals you can express numbers from 1 to 3999. There are other alternative forms where you can express numbers greater than 3999.
Splitting 600 we have: 600 => DC. We have to concatenate corresponding roman numerals from thousands to units i.e. DC. So the final result is DC
