The possessive form is used with nouns referring to people, groups of people, countries, and animals. It shows a relationship of belonging between one thing and another. To form the possessive, add apostrophe + s to the noun. If the noun is plural, or already ends in s, just add an apostrophe after the s.
3. to indicate certain plurals of lowercase letters” ~the OWL at Purdue. Apostrophes hold a space, like a friend saving a seat. Most of the time they are holding it for letters or numbers that would normally sit there, but sometimes they are used with plurals to hold a clarifying space between a lowercase letter and the s, so nobody gets confused.