iCloud is enabled by default. To disable or enable this feature, navigate to App settings, and tap on "iCloud".
Sync with data from iCloud
The latest data that is present in your iCloud will be loaded. Use this if you have a second device that was successfully synced with iCloud. Be aware that "Saved songs, musical preferences, and your purchases" of the current device will be overwritten with the data that is present in your iCloud. Be sure that your other device was connected to the internet. If you only have one device the better use option 2. Note that it may take a couple of seconds/minutes until data in the iCloud is updated.
Upload data from this device to iCloud In this case, data that is present on the current device will be uploaded to iCloud and synced on other devices (if you have other devices). Use this if you only have one device or if the data on the current device is up-to-date and should be synced on other devices. Note that it may take a couple of seconds/minutes until data in the iCloud is updated. Your other device will sync on the next app startup.
You can sync this data across devices:
Musical preferences (Except Auto-inversions setting)
Add-ons (In-app purchases)
If you work simultaneously on two devices, you will be asked if you want to sync changes between the devices.
If you lost any data (like saved songs) after syncing with iCloud, have a look here.
If you have lost any data after updating from app version 3.1.1 or lower to version 3.2.0 or higher, have a look here.