You can create pixel selections based on a single channel or combination of channels. This feature is available from the Channels panel.
Pixels are selected based on the contribution that the chosen channel makes to its colour value. If a channel only contributes partially to a pixel, that partial amount is selected (i.e. where a channel contributes 20% of the colour to that pixel, that 20% is selected).
In the Channels panel:
The selection is stored at the bottom of the Channels panel as a new 'Spare Channel' entry.