The panel on the left side of the "Selections" dialog displays the color and alpha-channels (saved selections).
To load any of those as selection, choose it in the list and apply the "Load Selected Channel" 
command (see screenshot). Or just double-click the channel's line in left panel.
The alpha-channels will be discussed a bit later. At this time, let's see why load the color channels.
The Red, Green, Blue and RGB channels let us select the image luminosity, that is, its brighter areas.
The most often used channel for the purpose is he composite (RGB or Grayscale) 
one.
By double-clicking the uppermost line in the Channels list, you load the brightest areas of the RGB channel, that is "of the composite image" or "of all visible layers". To ignore the luminosity of certain layers, just hide those before applying the effect.
As result, the image areas get selected proportionally to their lightness. (That's how "Hide Luminosity" script works.)
The most evident use of this method is extracting line art from a scanned drawing:
- Open the "Selections" dialog.
- Select the drawing (presumably, white) background by double-clicking on the RGB channel (screenshot).
- Clear or mask the selected area.
On the contrary, to select darker areas of your image, you load luminosity and then apply the "Invert Selection" 
command.
Sometimes, it's the only way to extract, e.g., soft shadows from a light background (as a rule, the Magic Wand and Quick Selection tools are pretty useless for this).
Furthermore, this B&W conversion method is also based on the image luminosity selection.
In some cases, you may want to select the luminosity of an individual color channel. Again, hide in the Layers palette the layers you don’t need, and then double-click the "Load Red Channel", "Load Green Channel", or "Load Blue Channel" thumbnail.
Let's make it clear that choosing a color channel in the "Selections" dialog doesn't deactivate the other channels (as it happens with the Color and Tone dialog).
NB Also see Loading Modes.
