Subscriptions are how your customers receive notifications via notifiers when something important happens on your status page. Most notifiers can have subsciptions, save for unsubscribable notifiers like Feed and Twitter.
If a subscriber does not wish to listen to certain event types (e.g. Notifier Tested, Issue Upcoming, etc.), then they can exclude these events with the Notification Level option.
By default, subscribers will recieve most notification types, except for Issue Updated, which is fired when an issue is updated, but not resolved nor reopened. If these defaults are not satisfactory, then the Default Notification Level can be changed on any subscribable notifier.
Subscribers can also choose the particular components they wish to listen for events on. By default, subscribers are subscribed to all available components.