Silently there are 2 more capture masters ("auxes") being introduced. In total there are now 2 stereo .wav files created during a recording with all tracks record enabled. E.g. recording from a RM32AI these are tracks/channels 33,34,35,36. In UC Surface for the RM's the channels 35 and 36 that are now apparently reserved for the extra capture masters appear as AUX3 + 4.wav after being recorded.

What was the thought behind the introduction of these extra capture masters?

In UC they are still not reserved/marked to be capture masters. I assume this may come in the next UC versions and other related applications.