In my school, everyone does 12 units in the Preliminary year, no one does accelerated courses and everyone mainly has 5/6 lines with 2-unit subjects, so basically 10 units are 5 2-unit subjects and for the Extension students, the '5th subject line' is 5A and 5B where you can choose two 1 unit subjects eg maths ext and eng ext or maths ext and business services or if you don't wanna choose any 1 unit subjects, then you can just choose another 2-unit subject such as modern history, geography etc
However in year 12, that stuff kinda goes out the window, some people do 10/11/12/13 units and the students with odd units or 10 units just have free periods, or if you have 13 units, means you have 'offline' courses where your 13 unit, usually extensions or senior science is done before school or after school, where you stay back and it feels tragic because you gotta stay back with the juniors lol
Hopefully that cleared some things up