I feel my experience may be useful here at this point in the thread
Hx very definitively seems to be a measure of internal resistance. That is, how difficult it is for energy to enter or leave the battery, relative to a new battery. At 100%, it performs just like a new battery. It's hard to tell what 0% is, but I have a good dang impression...
I saw a Leaf a short while ago that had 6/12 bars -- 50% degradation!
Yeah, half its battery capacity. https://goo.gl/photos/vZ3D252e7gNyU77r7
-- and it had 29.38% Hx.
The result of Hx? It could not quick-charge at all, basically. I think it only took in about 25 amps, max -- or, what is that, about 10 kW, out of 50 -- at just below 50% SOC. As it approached 66%-70% SOC, it slowed to about 7 kW, and eventually got below 6.6kW, then charging on a quick charger even slower than my 6.6kW Leaf's onboard charger.
So, the correlation between Hx and apparent internal resistance is very strong.
OTOH, the SOH figure precisely matched the bars on the dash - he'd just lost his 6th bar (which is what got our attention and came in to get a Leaf Spy report pulled from his car), and it showed 50% SOH, exactly half (6 of 12 bars remain, 6 of 12 bars lost).
So, SOH is clearly "capacity remaining" and Hx is "internal resistance". The lower your Hx, the less regen and slower quick charges you get -- also, the more sensitive it is to heavy acceleration. Accelerating hard on a low Hx will dramatically reduce your range (and heat the battery).
Hope this helps clear any remaining fog up.