Everyone in here has something different to say...
Anyways, the way I understand it..some schools call each class a "unit." For example...if my school were on the semester system, and I took a bio, math, ochem, and history class...that's 3 BCPM units (1 bio, 1 chem, and 1 math) and 1 "other unit." If I were to take 2 bio classes second semster along w/ orgy and painting, that'd be 3 more BCPM (2 bio, 1 chem) units and 1 more "other unit." So, at the end of the year, I'd have 5 science units, 1 math, 1 history, and 1 art for a total of 8 "units."