navindra One very basic thing I didn't understand — why is it the relative humidity that's important as opposed to the absolute humidity?
I'm far from an expert on this, but I think @ShiroKuro hit the nail on the head.
ShiroKuro If the RH is low, it’s easier for moisture to “move out of” the piano, i.e., for the wood to dry out, which causes the wood to shrink and in worse cases, crack.
This right here. Very cold air holds more moisture, so if you have 15 "units" of moisture in the air, in cold air it would be only 10% RH because the air can absorb more, but in hot air it would be 80% RH, because relatively, the air is almost at saturation.
This is important because the water moves in and out of the air, and in and out of things in contact with the air, depending on the moisture level between them - they are always seeking equilibrium.
So seasoned wood for pianos is typically around 40-50% RH. If the ambient air is lower RH, the air can hold more, and thus wicks moisture out of the wood, drying it. If the ambient air is higher RH, the lower concentration of moisture is now in the wood, and the wood draws the excess moisture from the air into itself. This is all irrespective of the actual amount of moisture in the air, it's all about concentrations seeking equilibrium.
That's why you can use a PLS with a heat bar to change the RH under the soundboard - if the RH in the air is low, the heater turns on, warms up the air, and that air can no longer hold as much water, so the relative humidity increases, and now the air is less likely to need to pull water out of the wood in the piano.
At least, thinking about it that way makes it easier for me to understand.