I don't know if everybody's keyboards work the same way but in mine when I press CTRL ALT + 1, CTRL ALT + 2 and CTRL ALT + 3, Photoshop selects "something".
I guess that in the first case it matches luminosity with opacity but I can't figure it out what's the other two.

I'm guessing here, mind you, but I suspect it's selecting the lumnosity of the individual color channels.

Depends on which tool is currently selected. Often the number keys represent a percentage (1 =10%, etc.). Using that combination will indeed change which channel you are looking at (turn on your channel menu and press those keys and you will see it change channels).
:wavey:

I think I got it. It has something to do with the spectrum.
It seems that CTRL ALT + 2 selects from red to blue and CTRL ALT + 3 selects from red to green.