Understanding Channels In Photoshop
Storing a Selection Mask as an Alpha Channel
When you make a selection mask in Photoshop, the mask is an 8-bit grayscale image. When you save a selection in the file, Photoshop has to put it somewhere, so it is stored as a channel.
You can try this yourself. First you need to make a selection with one
or more selection tools.
5. Now click Select > Save Selection and click OK in the dialog to save it. Photoshop will save the selection as a new channel by default.
6. Now the Channels palette shows the new channel. Photoshop assigns
the default name of "Alpha 1" to the new channel.