The "Devices" list shows all mobile devices (cell phones & tablets) and the apps (COYO App or COYO Classic App) on which your COYO instance has been logged into.
The devices only appear here if one of the COYO Apps was used but not if the user has logged into the mobile browser.

If you would like to delete a device, it’s not sufficient to just delete the app on the device. The device must be actively deleted from the list via the browser app. 
Once a device is deleted from the list, the end-user is automatically logged out of the device.

Actively saved devices also receive push notifications from the COYO apps if this function is also activated on the smartphone and your COYO is configured for push notifications.