Playback sources are used to identify a Pandora entity and are unique and permanent. For station factories it is used to create a station and for all other types it identifies a playback item.
Setting a playback source is more expensive than retrieving an existing source. For this reason it is best to use the playback current query instead of setting the source when a user session is started.
Some mutations also return the current playback item in the response. Use this instead of retrieving it with the playback current query.
In order to give a seamless playback experience to users, pre-fetching is available via the GraphQL API. Instead of using the playback current query, you can use playback peek to get the next track in the playback source. Take note that playback sources have a limited lifetime and cannot be stored for too long.