A subject and unit is the same thing. Each degree (or course) at uni has many different subjects that you have to complete to get that degree. Each subject is worth a number of credit points (a typical subject is worth 6 credit points). You need a certain number of credit points to get a degree.
An elective is a subject that is like an option in your degree. Most single degrees have electives so you can do subjects outside of your faculty if you want (some degrees you have to). It gives you a chance to branch away from your specific degree area.
The BBusiness requires you to do a total of 24 subjects (8 per year, provided you do all 6 credit point subjects). In your first year, you have a choice of some core electives (you choose 1 from a list of 2). Then in your 2nd and 3rd year you do your major, and then you can either do a sub-major or 4 electives, or do a 2nd major. A major is 8 subjects and focuses heavily on a particular subject area (like Accounting or Finance). A sub-major is usually 4 subjects and focuses on the main issues in a subject area only. A general elective in the BBusiness (if you don't choose to do a 2nd major) can be any subject in the uni (provided you're allowed to do it).
Hope this helps.
