In this instance, the second argument, here "slice", is what you want to actually be filled. As you have a stack, you have the option to fill all the images in the stack, or just the current slice.
If we take the above macro, we want to only fill the saturation channel (slice 2) with the white foreground color, not all 3 channels of the HSB stack.
It might be easier to try and follow these steps manually, as you can then visualise the corresponding dialog boxes etc...
After selecting the entire saturation channel, Edit > Fill would then produce a dialog box like this:
Clicking no would produce the following:
Clicking yes would produce the following:
Hope this helps!