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Mille is latin for 1000 and therefore the roman numeral for 1000 is `M`.

It's pretty unlikely for anyone to use a mix of roman numerals and decimals to describe.. Anything.

It's used in accounting in certain companies. It probably depends on the company culture.

Mille is sometimes used (eg, CPM is Clicks Per Mille/Thousand)

It's also sometimes used in the US for 1/1000 of a dollar for property taxes (mill levy).