Dear Mr. B:

You wrote:
> Hello,
> Please help with this grammar issue: What is the rule for using
> apostrophes with acronyms? I see apostrophes commonly used with an acronym
> to indicate a plural usage but doesn’t it really mean that it’s possessive?!?
>
> RMM’s
> RMMs
>
This is a good question. There is a little confusion because we DO use ‘s for plurals of words, letters, and numbers that name themselves: don’t forget to dot your i’s and cross your t’s. However, plural acronyms which have no periods are treated like words. They would only have apostrophe plus s for possession.

Example:
Possessive: He was appointed NATO’s presiding secretary.
Plural: He studied the REMs of 200 sleeping subjects.

I hope this helps.