Does a mask isolate changes to the selected area OR just display info for the selected area?

The mask will limit the 'touch' to the area inside the mask, unless you click the ECP option to invert. IF the mask is setup and working as you want, the rest of the image will disappear from the screen.

One way to do something similar to this is create an HSL key for skin tones where that's the only thing you've done in Lumetri, with the HSL key mask showing in color/gray mode. Export that as a .cube LUT or as a preset. You'll be adjusting the whole image, but according to the needs of the skin tone. To follow this for adjusting skin only, simply do this in the HSL tab of your current Lumetri instance. That's simpler of course, and affects only skin tones. To correct the color within the HSL, use the one/three-wheels setup or the color balance sliders.

The following shows the basic setup:

First, you need a clip with a proper skin tone. You can actually correct for proper tone in one Lumetri instance, then apply a second below that one, and do this procedure in that second instance. I sampled the area circled in the cheek ... then set for mask on (check-box) and color/gray. Then adjusted the Hue, Sat, and Luma mid-point/range/falloff settings to get the most of the skin without other things showing much, then did some denoise and blur to get rid of artifacts/chatter, and ... do your correction in the color controls of the HSL tab ...

or ... with the above keyed mask set ... and ... saved as a .cube LUT, or you can also choose a Preset of this for a slightly different following workflow ...

With the LUT, put in the tech folder you create to store Tech LUTs now according to the following chart  ...

... you can simply browse the Basic tab's Input LUT drop-down for this LUT you've named, and it will appear with the skin tone mask on. Check your skin tones in the Lumetri scopes & adjust ... if they fall outside the mask, adjust your color correction settings by your tool of choice until the skin falls back into the area you want.

Then simply remove the LUT from the Basic drop-down slot.

With a Preset, you'll want to probably apply this to Lumetri with either an adjustment layer and apply the preset to the AL, or apply it in a 'clean' Lumetri on the clip, then add a second instance of Lumetri, and do your work. Remove the AL or the earlier instance when you've got your skin adjusted.

Neil