This question was asked and closed last year as general reference. However, it did not attract the caliber of answer I expected it to. I suggested the following content as an edit, but it was rejected for "changing too much" (even though I believe "the original meaning or intent of the post" was well preserved). Additionally, "what is the correct abbreviation for millions, billions and trillions in a financial context?" touches on the topic from a financial news writing perspective without touching on financial documents. And this question admits to the commonness of M and MM, but merely concludes not to mix the two. Therefore, I am posting this question because I believe it would benefit from a canonical, thorough answer.
It is a common accounting practice to use Roman Numeral suffixes to abbreviate numbers that are in the thousands and millions. See this google search, which includes this article1 as the most authoritative explanation.
The Roman numeral M is often used to indicate one thousand, and MM is used to indicate one million. For example, an expense of $60,000 might appear as $60M. Sales of $3,000,000 might be written as $3MM. Internet advertisers are familiar with CPM which is the cost per thousand impressions.
In recent years some people began using k to represent one thousand. For example, an annual salary of $60,000 might appear as $60k instead of $60M.
In a recent business publication I saw million represented by mn and also by m (both lower case). This means it is possible for you to see $1,400,000 expressed as $1.4 million or $1.4mn or $1.4m or $1.4MM or $1,400k or $1,400M.
Wikipedia's article on Roman Numerals implies that this is no longer the case:
Roman numerals however proved very persistent, remaining in common use in the West well into the 14th and 15th centuries, even in accounting and other business records...
Even while the article on the number Million mentions it is still in use.
[One million] can be abbreviated MM in some financial contexts.
And neither article (or the references) provide any guidance to what situation each abbreviation is more suited to. The only hard rule I know of is the advertising world where cost-per-impression is typically abbreviated CPM for cost per thousand impression. Clearly, the first wikipedia article linked would be more accurate to say that Roman numerals persisted into the 21st century.
As an author, how can you decide when it is appropriate to use metric abbreviations or roman numeral abbreviations? When writing a quote for an order for products, would you write 25K or 25M to refer to 25,000 parts(or pounds or units)?
As a reader, how can you decipher whether a document is using metric or roman numeral abbreviations? If all the numbers are in the same range and abbreviated the same way (i.e. there are no occurrences of k or MM elsewhere), what other clues can you use?
1: Accounting Coach, accountingcoach.com by Harold Averkamp, who claims CPA and MBA status. Retrieved 2014-08-25 19:25 UTC.