For the purpose of determining order of layoff and reinstatement, an employee shall lose all acquired seniority rights in the following cases:

1. Upon resignation;
2. In case of a transfer that involves a change of title, unless the transfer is to a position previously held by the employee;
3. In all cases of reduction, unless the reduction is to a position previously held by the employee; and except as provided in 20.I and 23.D.