It's not a possessive marker. It's a plural marker. Please note,Originally Posted by hela
To avoid confusion, use apostrophe -s with lower case letters when you want to express plural number, for example,
EX: The word Mississippi has many s's and many i's.
With capital letters use apostrophe -s only if there is a possibility of confusion.
EX: As on a report card are wonderful. (Confusing)
EX: A's on a report card are wonderful. (Not confusing)
Whose CDs are these? (OK without the apostrophe)
Whose CD's are these? (OK, but the apostrophe is not necessary)