Yes, second master station. It comes up as M, the same as the first master in the web interface.
Is there any limitation to which station is the master? In the API doc (maybe an old one) the master could only be assigned to the first 8 stations. I was thinking of putting mine at the top end at 39 and 40. I was able to assign them in the web interface. Am I going to end up with any issues with the API? I don’t plan to use the API to change the stations but want to use the API to switch between master 1 and master 2.