Extensions are executable code bundles, in one app that perform
functions in a second, host app. Host apps declare extension
points that control the kinds of functionality its extensions can
implement. Extensions allow iOS and Mac apps to include code that
runs inside system apps. For example,
Messages
provides extension points so apps can create iMessage
Apps. Messages
automatically finds extension bundles that target its extension
points and makes them available in its app drawer. A Mac app can
also declare its own extension points so that other apps can extend
the Mac app’s functionality.