“That’s a tutorial for photoshop 7. What I call “Set of Brush” (brushes with different values of opacity and flow) is an easy way to caricature the best use of each brush (my use) this doesn’t mean this is the unique use. Of course, you have to mix the sets ( b, c, d…) when you draw. You won’t use one set by drawing!”
http://www.3dtotal.com/team/Tutorials/brush_away/brush01.php