CGSRegionRef: How is an arbitrary region represented as union of rects?

Question

The CGSRegionRef type is an undocumented/appkit internal CFType that seems to be able to decompose an arbitrary region into a union of rectangles. However, while the underlying APIs have been reverse-engineered for some time now, I'm having trouble actually understanding the debug output from NSLog'ing a region ref. For instance, the output of

       CGRect rect1 = CGRectMake(0, 0, 50, 10);
       CGRect rect2 = CGRectMake(0, 0, 5, 40);
       CGSRegionRef out1, out2, out3;
       CGSNewRegionWithRect(&rect1, &out1);
       CGSNewRegionWithRect(&rect2, &out2);
       
       CGSUnionRegion(out1, out2, &out3);
   

       NSLog(@"%@", out3);

is

#<CGSRegion 0x600003950630 [0 0 50 40]
         0 [0 50] 
        10 [0 5] 
        40 []
>

which appears to provide some sort of decomposition for a region composed of a 50w10h rectangle and a 5w40h rectangle both with their lower-left corners at the origin. However, it's not clear exactly what the format of the 3 rows is. They don't seem to be convex hull points because (0, 0), (0, 50), (10, 0), (0, 5) doesn't go all the way around the object.