Do you have a large subscriber base? As you probably know, until recently, trying to manage that list of subscribers has not been an easy task. Way back in the early days of MT-Notifier you had to do all your subscriber management through a series of drop-down menus. Then a rudimentary admin interface was created by Blumunky, and finally back in version 3.5.0, I added an actual, honest-to-goodness interface to the plugin itself. But as you have likely discovered, paging through a number of entries (thousands, hundreds, or even dozens) to find one of those subscribers is still a bit unpleasant, and it's just not ideal.
With recent versions of Movable Type, it became possible for plugins to register their own object types (like subscriptions). Those objects could even be searched based on registering a search API. But until recently, I had been unable to crack that code (sometimes the code is, shall we say, less than well-documented). So if you had tried to use the Search Subscriptions box in the upper-right corner to actually help speed the process of finding a subscription in your monstrous list of subscribers, you would be met with an ugly error message. No more.