I uses HB graphite pencils when I want to draw sharp lines and the surface of my art is very hard. I also use 7B or even 8B when I want a thicker, softer line or texture on top of my acrylic painting.
The 'H' stands for hardness, the 'B' stands for blackness, and HB is for hard and black pencils. The hardest is a 9H, followed by 8H, 7H, 6H, 5H, 4H, 3H, 2H, and H. F is the middle of the hardness scale; then comes HB, B, 2B, 3B, 4B, 5B, 6B, 7B, 8B, and 9B, which is the softest.