The Ghost Copy 21 App is an optional extension of the Online Challenge 21.
The Online Challenge 21 also works without this app.

With the Ghost Copy 21 you can:
Ghosts of the Online Challenge 21 can be read, inserted, saved and shared.
You also have access to the ghosts from the archive.
If you play Online Challenge 21 without an Internet connection, you can save your ghosts with the GC:21 and upload them later when you have an Internet connection again.

For more information see https://ski-offline-challenge.ch