Buses are Included in the Premium Subscription of Playback. Playback includes 10 buses. Using SmartRouting every track in our catalog is pre-routed to 1 of 10 buses. When you plug in a supported USB Audio Interface SmartRouting will route all 10 buses to the outputs on your interface. You can edit Bus Names and Abbreviations (what they show up as in the app) by swiping to the left. You can also adjust bus-output routing at any point by tapping bus in the Buses page. While in-app you can tap EDIT and tap on the name of the track to change track-bus routing or bus-output routing.
It's important to note that Mute, Volume, and Track-bus routing can edited for every song, but Bus-Output routing and soloing is a global function that will effect all tracks.