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$1m == $1MM == $1mn == $1 million == $1,000,000.00

It's weird.

The MM comes from the roman numeral M which means thousand, so one thousand - one thousand == MM.

For SEO guys, you'll now understand why it's called CPM vs CPK or similar.

Not in the parlance of accounting.

Sounds right, though it's confusing when many people use this notation differently. Is there any standard? though, I guess, it's off-topic.

True, yet further down the article we see: "What does $1M buy you?", which must be a typo.

No it does not.