After the ring has been paired with one device, it protects the collected data from outsiders with an encrypted connection. If someone tries to reconnect the ring with another device, the app will require the user to perform a factory reset to erase any collected data from the ring memory.
If the app notifies you that it's in Restricted mode, you'll need to reset the ring to factory settings in order to take the ring to use.
Please note that only the data from the ring memory will be erased. Data that has been synced to Oura Cloud will not be affected.