Mileage policies are used to define how much an employee is paid per mile when claiming mileage expenditure as part of their expense claim.
You may setup as many policies as you need in the system. Each policy describes the per-mile rate depending on the vehicle chosen, the date the mileage was incurred and the total amount of mileage claimed to date in the year the policy is assigned to.
To illustrate, you may have a policy that states that, for car drivers, any mileage recorded after 5th April 2001 is paid at 45p a mile for the first 10,000 miles, then 25p a mile after the first 10,000 in any tax year. The same policy can also contain mileage rates for other vehicles.