Your RWTC subscription limits the number of devices your login can use at one time. Whenever you log in on an app, it is recorded as an active device.
When you log-in on more devices than defined in your subscription, all devices will be locked until one or more devices are removed (set inactive) on this page or you adjust your billing agreement to increase the number of active devices.
You can see how many devices are permitted below.
Songs from RWTC will become inactive in OnSong until the number of active devices is within your agreement.
It is recommended you rename your device(s) so you know which ones are active.