Gmail - read and unread message status options for email and conversations

In Gmail, if you mark an email as unread it will become bold and display the following notice:



If you try to mark an email as read that's already read you'll get a notice but obviously nothing happens since the email is already read:



There are both buttons and/or menu options to mark email and conversations as (un)read including:



It is also possible to pick a message within a conversation and mark unread from a point within a conversation: