Some labels such as ‘deleted’ and ‘filed’ have built-in meanings, and Rmail assigns them to messages automatically at appropriate times; these labels are called attributes. Here is a list of Rmail attributes:
Means the message has never been current. Assigned to messages when they come from an inbox file, and removed when a message is made current. When you start Rmail, it initially shows the first message that has this attribute.
Means the message is deleted. Assigned by deletion commands and removed by undeletion commands (see Deleting Messages).
Means the message has been copied to some other file. Assigned by the o and C-o file output commands (see Copying Messages Out to Files).
Means you have mailed an answer to the message. Assigned by the r
rmail-reply). See Sending Replies.
Means you have forwarded the message. Assigned by the f command
rmail-forward). See Sending Replies.
Means you have edited the text of the message within Rmail. See Editing Within a Message.
Means you have resent the message. Assigned by the command M-x rmail-resend. See Sending Replies.
Means you have retried a failed outgoing message. Assigned by the command M-x rmail-retry-failure. See Sending Replies.
All other labels are assigned or removed only by users, and have no standard meaning.