100 level are generally first year subjects (no-prerequisites) and 200 level are second year. But you can do 100 level subjects no matter how far through your degree you are.
100 level are generally first year subjects (no-prerequisites) and 200 level are second year. But you can do 100 level subjects no matter how far through your degree you are.
However there's generally a limit on the number of 100 level subjects you can do.
200 level subjects will generally have a certain number of 100 level prerequisites, and 300 level will have 200 level prerequisites. Note though that some universities and some faculties have removed a lot of 300 level subjects and you only do 100 and 200 level (so you basically do two years of 200 level subjects). (eg history at USYD).