It's just a normal pencil brush.
It isn't very transparent, but it has a
gentle, fuzzy look to it.
I don't have any special instructions on how to use it, so here's a size comparison.
(In order, top to bottom : 5, 10, 15, 20, 30, 50, 80, 100, and 150)
It is a very versitile brush! To show you it's uses,
here's an illustration I did using only this brush!
(Yes, even for the watermark.)