The Device Notifier is a plasmoid that is responsible for handling device events. When new data device is inserted or detected by Solid, it gets added to the Device Notifier menu. Each device has a set of actions the user can perform on the device, depending on the device type and its contents. For example, when the media is an audio CD, the Device Notifier offers you to play that CD using Amarok. The default actions are not a part of Plasma; each one is installed by the application that handles the action, and goes away when the application is removed.
You can add your own actions and modify existing ones using the Device Actions system setting module; however, such changes are limited to the user that requests the change. If you want to install a custom action along with your application, you have to dig a bit deeper.