Keyword field (100 characters)

It is invisible to users; it only enters the search index. A hundred characters is a hard limit and every character is inventory.

The three most common ways to waste it: leaving a space after a comma (Apple splits on the comma, the space indexes nothing), repeating a word that is already in the title or subtitle (Apple indexes those too and combines terms across fields), and writing both the singular and the plural of the same stem.

rankcusp's Metadata Audit screen counts all three and shows how many characters can be won back.