I have been trying to understand where the azimuth angle references from. For example, I ran Test26.fst which its ElasoDyn has initial zero azimuth angle, but the AeroDyn output is 90 degree on blade 1 azimuth angle at initial.
I read the AeroDyn Theory Manual (from Dec. 2005), and found this...

But it seems to be something else.
Can someone verify on the AeroDyn v15.0 azimuth angle outputs , "BαAzimuth", about where is its reference point?
Is it on the Y-axis or Z-Axis of the wind-turbine?
Thank you all.