The idea is to be able to use a device as an extended display for another device. The app would run on the extended display and would behave as though it was a Varia Vision display using the recently announced extended display ANT+ protocol.
For example the athlete has a Fenix 5 and an Edge device, and wants to use the Edge device as an extended display for the Fenix 5 which is used to log all activities.
An app like this might also provide a motivation for Garmin to implement the extended display protocol across a wider range of devices.