A Custom ringtone is a feature available in Item Settings of your TrackR app. Selecting "Customize Ring" you can choose, from your media library, the song you would like your phone to play when you either ring your phone from your TrackR or when using Separation Alerts. If you are confused why the TrackR app asks for access to your media library, it is related to using this feature.
It is important to note that Custom Ringtones is only an option for the phone's ringing. The TrackR's ringing cannot be changed, either in tune or volume.