The ATAR is a rank, not a mark
The Australian Tertiary Admission Rank (ATAR) is a number between 0.00 and 99.95 that indicates a student’s position relative to all the students in their age group (ie all 16 to 20 year olds in NSW). So, an ATAR of 80.00 means that you are 20 per cent from the top of your age group (not your Year 12 group).
Universities use the ATAR to help them select students for their courses and admission to most tertiary courses is based on your selection rank (your ATAR + any applicable adjustments). Most universities also use other criteria when selecting students (e.g. a personal statement, a questionnaire, a portfolio of work, an audition, an interview or a test).