When a list is using shared settings, there are still a few settings that can be unique per-list. If you are trying to edit the settings for an individual list, please make sure you are editing the correct list! The one you are editing will be displayed at the top of the page.
The List Name and List ID are, of course, always unique. You can change those settings at About This List.
Lists with Shared Settings can also have unique Administrators. A list may wish to designate a subscriber to that list as an additional listowner. Or there may be subscribers who should be able to send mail or moderate traffic. (Account Administrators are always shared by all lists on an account.) Administrators can be added, removed or edited at Administrators.
Finally, the free-form text portion of the message footer is unique per-list. If your shared posting policy is set to Listbox Composer Only, you can edit this text portion in the Composer when you are writing your message. Lists whose posting policy is set to accept email can edit this setting at Message Format.
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