Much like the AUTH DECLINE command this command will prevent you from being added to someone's buddy list, but the other person will not be notified.
Example:
/msg nickserv auth reject 1
You may wish to reject a buddy list request rather than decline it either if you do not want the person informed that you rejected it, rather than simply failing to respond. Another reason to use reject is if the request was sent by accident or you spoke with the person who sent it so that no memo would be necessary.