Some more details about Phone Cover apps.
OEMs can build and register a phone cover app, which allows partners to create a user experience with their Windows Phones phone cover accessories. This application must be preloaded on the phone as a Settings/CPL application.
OEMs can then enable the app to be launched when the phone cover is closed and specify the default setting for the lock screen’s auto unlock setting, which determines if the lock screen is automatically lifted when the user opens the phone cover.
Limitations and restrictions:
When the OS receives a notification that the cover state has been set to Closed:
The OS locks the device.
The OS uses the AUMID setting to launch the phone cover app. The phone cover app is launched in the foreground, above the lock screen, and the app is rendered at the top of the z-order.
When the OS receives a notification that the cover state has been set to Opened:
The OS terminates the phone cover app and shows the default lock experience.
If the user opens the cover and the AutoUnlock setting is set to 1, the OS automatically lifts the lock screen and tries to unlock the device. If the device does not have a password lock, the OS unlocks the device. Otherwise, the OS prompts the user for their password.