The numbers on a pencil indicate the hardness or softness of the ‘lead’ – or the core of the pencil. This core is made from graphite & clay. The more graphite in a pencil the softer or easier to make a dark line. The more clay- the harder & lighter (and more precise).
So a 9B is a very soft pencil lead and a 9H is a very hard one. In the middle of those two extremes is the HB (sometimes called F for fine). Something like this:
I am sure that there are other markings on different brands but these are the ones that I see most.
The one that you are asked to bring to your SATs is a 2B….the most typical school pencil. It is very close to a 2B drawing lead…and a good one to use for drawing as long as you can erase it cleanly.
A 2B (or any of the others) drawing pencil will cost between $1 and $3 in most store.