The top half of that page is to notify people when you’ve added items that are
on their Wanted Lists. It notifies everyone that have opted in in their Wanted
List. Only them, and only for the items that match their WL (qty, condition,
price…).
Subscribers: People can subscribe to your “store mailing-list.” They do it the
same way they favourite your store.
The message you write there is sent directly to your subscribers but it’s also
posted on the forum by default. You need to remember to uncheck the option at
the bottom to prevent a post on the forum if you don’t want to fall fool of the
forum rules (https://www.bricklink.com/message.asp?ID=1437060 ).
USA, California
France, Nouvelle-Aquitaine