If you look at the modal, it reads
Category Slug
(Optional) dashed-words for url
Not a problem as long as someone knows what format slugs have.
Not a problem as long as someone knows what is and isn’t a word.
It seems seeing “dashed-words” would be clue enough.
And coders would understand [-a-z]+
But I can’t think of any way things could be made more clear so that everyone would understand what it does.