In this panel you can choose if Users are allowed subscribe to email notifications related to Comments.
For Registered users, they may be allowed to subscribe to notifications for every new comment in the collection or to a specific post.
For Anonymous users, they may be allowed to subscribe to notifications for replies to their comments. With the Default option, you can select whether the option for anonymous users to subscribe to replies is checked or unchecked by default. You can also set the maximum number of emails anonymous users may receive per day with the Limit option.
See also Subscriptions Panel for more options.