Unless the -U switch is used it will switch its user id to the user id owning the list directory. This is done to make sure that new files created are having correct permissions.
The nomail version of the list is a list version where people are subscribed like usual, but they won't recieve any postings to the list. This is useful for people who read the mailinglist through a news gateway, but want to be able to post to the list.
Normally a mail is sent to the subscriber if the address is already subscribed to the list. If the -s switch is used such a mail will not be sent.
When neither -c nor -C are specified, subscription silently happens.