As you know, facebook makes your friend request hidden when you mark a friend request as "not now" but there's also an extra option added to "not now". That is, it displays a question, asking you "Do you know (Name) outside of Facebook?" and gives you two option "Yes" or "No". If you confirm "yes", then it'll keep the request hidden and later you can accept the request. But if you confirm "no", then it may resist that user for future friend request.