The Channels drop down lets the user choose which channels are used for the display in the Canvas view. This does not affect the data being used to light the view in the Render View, or the HDRI map being shared with a Connection. It simply controls the display as seen in the canvas view.
The full list of Channel choices are:
HSVA - this is the default choice as it makes the user aware of the areas of the Canvas that are transparent.
RGBA (Pre_Mult) - this view multiplies the RGB (or HSV) content by the alpha and display the resulting HSV image.
Here are some examples: