M is 1000 in Roman numerals. While it is uncommon, M is used to mean 1000 in business contexts, such as CPM: https://www.investopedia.com/terms/c/cpm.asp https:

M is 1000 in Roman numerals. While it is uncommon, M is used to mean 1000 in business contexts, such as CPM: https://www.investopedia.com/terms/c/cpm.asp https://www.investopedia.com/terms/c/cpm.asp Therefore "mm" is the most unambiguous way to denote millions, and is used in accounting.