Notes is an inside messaging system on DeviantART. You can send one message at a time or you can send same message to many people, but all receivers will see others.
Inbox
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Inbox is a notes folder where all your notes will go until they are deleted.
Unread
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Almost the same as inbox but here you can see only the notes you have not read yet
Starred
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In the starred section are stored all notes that you have starred. You can use this to mark important notes.
Sent Notes
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Sent notes section is a section where your sent notes are stored until they are deleted.
Drafts
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You can save your notes as a drafts and they will be stored in this section so you can use them later if you want to.
Spam
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You can mark sender as a spammer and all notes from him later will go in this section and will stay there.