Tax on travel allowances

Pls explain the difference between the 80/20 and 20/80 tax principle for travel allowances and when can it be used ?

Is it for company cars or travel allowances?

If the employer is satisfied that the employee is driving more than 80% of their time for business purposes then you would only include 20% of the travel allowance when calculating how much PAYE to take off. If the employee travels less than 80% then you would include 80% of the travel allowance in calculation the of PAYE.