Roman Digits are used to express Roman Numerals. Commonly known are the digits I, V, X, L, C, D and M. But more digits do exists, although they are not used in these days where sometimes Roman Numerals are used as (for example) index page numbers. Following digits exist.
If a horizontal line is found above one or more digits, the found value must be multiplied by 1000. And to express even larger numbers, one can embed the digits in a rectangle, with the bottom line missing, meaning that the value has to be multiplied by 100000.