1. To reimburse employees for business travel, or
2. To calculate the amount that employees should repay for any private use of the vehicle

AFRs are reviewed quarterly at the start of March, June, September and December, though employers can choose to the old rate for a further month. The rates from 1 December 2017 are shown in the below tables comparing them with those that applied in the previous quarter. I have highlighted the changed rates in red. As you will see, there are four changes:

All of the above information can be verified on the Gov.uk website where it is contained in four separate documents.