Hi Leah,
Severance Pay is base on ‘Basic Wage’
Basic wage is mean the money that the employer and the employee agree to work with the employee. In a normal working day and time, such as salaries, wages, daily living. Value, location, etc.
But as your concern to wording that is mean income non-fix to pay to the employee for a benefit , for allowance. It pays temporary. It isn’t basic wage.