When a postgraduate student does not return for a new academic year, you should submit a Withdrawal CoC. This should be in the previous academic year, with the last day of the previous academic year as the effective date.
For example, a student was in attendance for the whole of academic year 2020/21. They do not return for academic year 2021/22. You should submit a Withdrawal CoC for 2020/21. The effective date will be 31 August 2021, as that is the last day of the academic year for this course.
You should not submit an N registration code for a non-returning postgraduate student, as this will cancel their application for every academic year.