Decimal numbers are a base 10 numeral system.
In Roman calculation, stones are placed on lines representing units, tens, hundreds and thousands. The spaces between the lines are used to represent intermediate values that is 5, 50, 500.
Carrying: So it is never necessary to use more than 5 stones on a line or more than 2 stones in a space. This is because 5 stones on a line can be replaced by one stone in the space above and 2 stones in a space can be replaced by one stone on the line above.
Borrowing: Or the reverse a stone in a space can be replaced by 5 stones on the line below or one stone on a line can be replaced by two stones in the space below. Zero is simply represented by an empty line or space.