What parameters can B.U.D.S. access?

I think you are asking if the Rotax CAN bus monitors Alternator B. The answer is no. You need to monitor the airframe electrical system separately so that you will know if Generator B is connected to the airframe and providing power.  A volt meter on the airframe buss will generally tell you this by the fact the voltage is being kept up to full charge, but many also want an amp meter because it’s a more direct reading.

Depending on where you install the amp meter (shunt) you can get different results. If the shunt is installed between Alternator B output (X3-3) and the airframe power bus, that will show you the total amps being supplied to the airframe and battery by Alternator B. However it is more common to install the shunt between the battery and airframe bus, as in the attached diagram. This configuration shows you the amps being supplied to charge the battery, which is amps in excess of what is required to run the airframe and may be negative or positive depending on airframe loads and the battery state. Typically you see a higher reading right after Alternator B is switched over to the airframe, then that reading decreases as the battery is recharged from being used to start the engine. If alternator B fails and there is no aux alternator, you would expect to see a steady state discharge (negative number) from the amp meter.

The attached diagram is from the 912iS installation manual. I think this is more clear than the diagrams provided in the 915 installation manual.  

Yes, that is my question.  I have a dual battery/dual bus architecture system and have current monitors on both buses where they enter the cabin so I can measure the airframe current draw.  Obviously, if the battery is charged, then the current drawn by the airplane should equal that produced by the generator.  However, I was thinking it might be good to know the actual generator output to see how it is working directly.

I was hoping that the Rotax engine computer might monitor that, but thanks for confirming that it doesn’t.  And, yes, monitoring bus voltage will tell if there is a gross issue with generator B.