Colour zones in darktable 3

Well, it’s a less-than-3-minute video about a rather capable module.

Half of the playtime is about the trivial things: 3 tabs and what they do.
Select by area and the colour pickers are covered between 1:12 and most of the explanation is over by 2:15.

This is what the module looks for one of my shots (the shot itself is not important):
image

According to the colour picker’s tooltip, I should be able to select an area, but no matter if I clicked with Ctrl on the picker, or with Ctrl on the the image, I was unable to do so. And what would such a selection actually achieve?
image

For the next screenshot, I switched the module into ‘select by lightness’ mode and switched to the lightness tab, as the point is more visible. Selecting with the ‘create a curve based on an area’ mode, I see that new control points are defined.
image

If I use Ctrl or Shift while selecting an area, I get a curve which will be positive or negative at the mean(?) value, respectively, but otherwise looks quite weird:

Then comes ‘edit by area’. If I Ctrl+click, I don’t just get a point: I get some value assigned to it, I have no idea based on what (I clicked where the pointer is visible):
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What do those light grey ‘envelopes’ represent? They can look quite weird, just look at Rico’s video at 1:35.

Well, these are my questions.

Thanks,
Kofa