Each list user (E-mail address) is subscribed in one of the following modes:
FEED: In this mode, the subscriber receives list messages as they are posted. See the FEED Mode Distribution section for more details.
DIGEST: In this mode, the subscriber periodically receives digest messages. A digest message starts with the Table Of Content - the list of the messages posted, followed by the posted messages themselves. See the DIGEST/INDEX Mode Distribution section for more details.
INDEX: In this mode, the subscriber periodically receives index messages. An index message is the same as the digest Table Of Content, but it does not contain the posted messages themselves. INDEX subscribers can see if they are interested in any posted messages, and use the Web interface to read those messages in the mailing list archive. See the DIGEST/INDEX Mode Distribution section for more details.
NULL: In this mode, the subscriber does not receive any messages from the list. This mode can be used by the "posters" - the list users that only post messages on the list.
BANNED: The "banned" users do not receive messages from the list, and they cannot change their subscription mode themselves. You can use this method to make it impossible for certain users to subscribe to your mailing lists, though usually the more generic anti-spam and other system-wide protection methods should be used.