What does such statuses as Open and Closed mean in Notifications section of In Stock Reminder app?

A status closed have notifications with a message, which has been already sent to a customer, that a price for this product has been decreased. Also a status closed is supposed to be set in a case, if a notification was created by a customer for a limit period of time (for example, that notification is actual within two weeks) and this time has been expired already. Otherwise, notifications have status open .