Some labels such as deleted and filed have built-in meanings and are assigned to or removed from 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 (Section 29.4).
Means the message has been copied to some other file. Assigned by the file output commands (Section 29.6).
Means you have mailed an answer to the message. Assigned by the r command (rmail-reply). Section 29.10.
Means you have forwarded the message. Assigned by the f command (rmail-forward). Section 29.10.
Means you have edited the text of the message within Rmail. Section 29.15.
Means you have resent the message. Assigned by the command M-x rmail-resend. Section 29.10.
All other labels are assigned or removed only by the user, and have no standard meaning.