@gs5556 Your explanation is partially correct. The bit about 'partial triode' is not. What UL allows is for a pentode to have better linearity than a triode.
However there's been a wrinkle caused by the fact that the UL technology was the topic of a patent. To get around it, other manufacturers moved the taps away from the ideal point as taught by the patent. The linearity goes down rapidly. The patent also taught there was an ideal percentage for the tap that depended on the power tubes being used, which is why you can't use the same output transformer with EL34s and 6L6s unless there is a tap for each tube type.
The use of the incorrect tap has become a tradition and is now so ingrained that everyone thinks 40-43% is the right ratio; so UL operation it typically incorrect. I think that's part of why you see debate around 'UL vs triode' in high end audio. Depending on the amp, triode may or may not be better as a result. So its good in the case of the original post that the Cary seems to have gotten this bit right.