To apply for a Commonwealth supported place (CSP) or Higher Education Loan Program (HELP) loan you need a Unique Student Identifier (USI). You also need a USI to graduate at the end of your degree.
The USI was introduced into higher education on 1 January 2021 and replaces an earlier individual identification number called a Commonwealth Higher Education Student Support Number (CHESSN).
Depending on when you started study you may have a CHESSN, a USI or both.