Unmoderated means that all messages are automatically forwarded to the entire mailing list. Open means that list subscriptions are granted automatically by the list processor. Approved means that the list owner must manually approve each subscription request. Open means that anybody in the entire universe of electronic mail will be able to submit messages to the list. Subscribers only means that only list members may submit messages to the list.
Owners only restricts posting to the list owner.
This is suitable for a list where replies in general should be seen by all members. Which one you use depends on the way you want to run the list. Forward to owner means that messages to the list from non-subscribers are forwarded to the list owner for handling. Return to sender means that a rejection notice is sent directly to the sender without involving the list's owner. List owners are encouraged to consider the benefit of automatic subscriber deletion.