Conceal: check this box to report on whether or not subscriptions are listed in a "Review" of the list of subscribers. This setting is only of interest for lists where the list of subscribers is viewable by other list members or the general public.
This setting is mostly superfluous on lists that are set such that only the list owners can review the list of subscribers, because list owners can always see the subscriber addresses regardless of the "Conceal" setting. However, list owners may set the list such that "Conceal" is the default option for all subscribers to indicate that subscriptions are allowed without requiring the first and last names of the subscriber.
See the LISTSERV Manuals for complete documentation.