> An expense of $60,000 could be written as $60M. Internet advertisers are familiar with CPM which is the cost per thousand impressions.

> The letter k is also used represent one thousand. For example, an annual salary of $60,000 might appear as $60k instead of $60M. [0]

k and MM are unambiguous. You don't want to stop & think, and you definitely don't want to be off by a factor of 1000. So it makes sense to avoid typing 'M' if you have any risk of being misinterpreted.

It's mostly about the reduction of risk, it's VERY rare to see M used in this way.

[0] https://www.accountingcoach.com/blog/what-does-m-and-mm-stan...