3. Basics of PhotoLine / 3.3 User Interface Elements
Many image tools (see chapter 6.3.1 and following) and filters allow you to limit their effect to certain color channels:
Here you can control which color model you want to use, usually the color space of the image, HIS or Lab.
You can also change the color channels affected. Clicking on a channel while holding down the Alt key has a special meaning. If other channels are active, they will be deactivated, making the clicked channel the only active one. If it was already the only active channel, all other channels will become active as well.