Hi Eric,
I figured it out at one point but it is too hard to explain. I suggest you create a black and white square bitmap with text in it, perhaps something that says
This is
a repeating
pattern
Set it as the grain.
Then using a brush head that you understand very well, or is maybe a single solid circle, start playing with the grain settings. You'll notice the text will "modify" the application of your brush head (sort of like a stamp or as they call it a brush "grain") in different ways.
This is pretty useful because it is like a third feature in addition to the brush head and the grain of the paper.