You should not submit a Withdrawal CoC for undergraduate students who complete one academic year but do not return for the next. This is because undergraduate students must apply for each academic year they want to attend. If the student does not want to attend the next academic year, they will not apply for funding for that year.
For example, a student completes the 2020/21 academic year. They do not return for the 2021/22 academic year. The student's application will not roll over into the next academic year. Therefore, you do not need to submit a Withdrawal CoC for 2020/21.
If the student does apply for the new academic year but does not return, you will need to submit a D attendance code. This will let us know that the student never registered. See Confirming ongoing attendance for more detail.