I’m in finance and exclusively use the M and K to reference millions and thousands. Everyone says the MM is more accurate and I get that from a Roman numeral perspective it may be (if you ignore that it actually means 2000), yet I’ve never once encountered anyone using M as a thousand in the writing or reading of financial figures. I think it’s primarily a financial news, journalism/writing style, I rarely see it in business at all. Sometimes I see MM from the PE/banker guys and that’s about the only time I see it IRL.