As I understand it, which admittedly may be incorrect…
Also, I apologize if you already understand this.
The “B1” is not so much about “BUS” as it is about “BANK”.
The midimap contains these lines:
<DeviceInfo bank-size="8"/>
<!--
<Binding channel="1" ctl="115" action="Transport/Rewind"/>
<Binding channel="1" ctl="116" action="Transport/Forward"/>
-->
...
<!-- Bank Control (the MPK doesn't have those, use the Rewind/Forward buttons instead) -->
<Binding channel="1" ctl="115" function="prev-bank"/>
<Binding channel="1" ctl="116" function="next-bank"/>
The “bank” function allows the user to control many more than just eight tracks.
The "bank-size parameter indicates that whenever the bank select buttons (Rewind and Forward) are pressed, the “bank” changes.
<Binding channel="1" ctl="70" uri="/route/gain B1"/>
When Ardour starts, “B1” refers to the first audio/midi track or buss", B2 would refer to the next, and so on. So, turning knob 1 would affect level of the first audo/midi track or bus, knob 2 the second, …
Master is handled differently as noted in the map by the line:
<!-- <Binding channel="1" ctl="7" uri="/bus/gain master"/> -->
If you were to press the Forward button, the “BANK” would move by eight tracks. Now, the “B1” binding would affect the 9th track in the session, and so on.
If the binding were targeted at “S1”
<Binding channel="1" ctl="70" uri="/route/gain S1"/>
then the encoder would affect the first SELECTED audio/midi track, regardless of which bank was selected.
If the binding were targed at “1”
<Binding channel="1" ctl="70" uri="/route/gain 1"/>
then it would affect the FIRST audio/midi track regardless of either bank setting for track selection.