Click on the “Leave Days” tab on the left menu.

On the leave days tab, click on the “Policies” and then select the rule which was assigned to the employees.

Click on the leave type which you want to set seniority.

Proceed to the "Seniority" step.

Click on the "Add Seniority Right" button.

Add additional days for employees seniority level.

Select the type's other settings from the next steps if you want then, click on the "Next" button from top of right and save changes.