I read the whole manual and tuning guide and I was either too dumb or inexperienced, or both, to figure out the difference between cable-actuated compression dial (black) and the manually adjusted dial (blue).
I think I have figured out rebound adjustment.
The manual has no pictures of my actual shock, but more generic descriptions. I suspect the blue knob is low-speed compression damping, whereas the cable-actuated black dial “threshold” something or other, which I have interpreted as “lockout, 30% stiffer, and fully open” for the 3 positions.
Weirdly, the RockShox tuning guide PDF has detailed discussion and diagramed explanation of how sag, rebound, and low-speed compression (LSC) damping work (pictured below), but no detailed discussion of how the “threshold” lever works.
This is all that’s said about the threshold feature, which is odd, because it’s the one that is most likely to be used most frequently, given the remote availability:
It seems like the threshold adjustment is like “soft, moderate, locked out” but they say nothing about increased stiffness throughout the full distance of shock compression, which their definitely is when the adjuster is in the threshold (middle) position.
Does anyone have any differential information between the LSC dial and the threshold dial on the rear shock? Looking for nuanced discussion about how they work for my understanding. Is the threshold remote-actuated 3-position dial how one tunes High-Speed Compression?? If so, I’m sure I can read a useful article on HSC vs. LSC and get this all sorted out.
I’m getting handy enough that I could take the thing apart and probably figure out what they do and why, but I’d prefer not to disassemble the shock for investigatory purposes!
Yep, this is precisely how I learned… except I just maxed it out in either direction then zeroed in on what I liked.
I’ve been taking detailed notes. 
Yep, definitely been doing this outside of what is specifically noted in the images above. Following the scientific process for sure 
(ie. change one variable when I really want to learn something)
Heard! (I watched RockShox youtube videos on fork & shock servicing yesterday, among other servicing videos, so feel capable here.)
Fascinating and sounds quite valuable. Googled it. Awesome. This seems like a no-brainer install on most bikes these days. Getting one now.