SAM stands for Student Assessment Modeller. Basically, it gives you a prediction of the UAI you would have received if you had completed the HSC in 2001, 2002 or 2003. You don't have to download it; it is on the main page of bored of studies- boredofstudies.org. Most people tend to place their current assessment marks into SAM. However, I wouldn't rely on it for an accurate prediction. It just gives you an indication based on previous years.