The choices are:

Mail

You receive copies of messages posted to the listserv immediately. Digest Each night, around 12:00 am, you will receive a single email message containing all the messages contributed to the listserv that day. At the top of the message will be a numbered list of the subjects in that digest, followed by the complete messages themselves.

The same as a digest, but in MIME format so that the individual messages' formatting is preserved. Some email clients such as Outlook will show the digest as a series of attachments.

Index

Each night, around 1 in the morning, you will receive a single email message containing all the subject lines of all the messages contributed to the listserv that day. If any of the messages interest you, the bottom of the index gives the email command that will retrieve the bodies of the messages.

Nomail

No email is sent to you. You are free to go the web interface whenever you want, and read the full text of the messages there. This setting is also useful for people who want the ability to contribute to a mailing list, but do not care to see the contributions to the listserv.