2u is advanced maths, 3u is extension 1 and 4u is extension 2. If you do 2u you only do the topics needed for 2u. If you do 3u you do topics studied in 3u and 2u. If you do 4u, 2u is assumed knowledge but you only do exams for 3u and 4u for the HSC. I don't do 4u but a few of my friends said...