Comment by Someone

Certainly in modern use CDI is “a hundred less than five hundred, plus one”, or 401

https://en.wikipedia.org/wiki/Roman_numerals#Standard_form:

“The numerals for 4 (IV) and 9 (IX) are written using subtractive notation, where the smaller symbol (I) is subtracted from the larger one (V, or X), thus avoiding the clumsier IIII and VIIII. Subtractive notation is also used for 40 (XL), 90 (XC), 400 (CD) and 900 (CM). These are the only subtractive forms in standard use.

[…]

While subtractive notation for 4, 40, and 400 (IV, XL, and CD) has been the usual form since Roman times [citation needed], additive notation to represent these numbers (IIII, XXXX, and CCCC) very frequently continued to be used, including in compound numbers like 24 (XXIIII), 74 (LXXIIII), and 490 (CCCCLXXXX).[12] The additive forms for 9, 90, and 900 (VIIII,[9] LXXXX, and DCCCC) have also been used, although less often“