Why does the VAT for a mileage claim not match the rate shown?

A mileage claim will use a Mileage Class, and these can be set to have a lower VATable Rate than the 'Claimable' one. Mileage Class settings can be reviewed by an administrator via the Settings area.
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The VATable rate means that the VAT rate for the expense is applied to the value entered per mile. Therefore, if the VATable rate is set as £0.10 and the VAT rate is 20%, VAT will be calculated at £0.02 (2p) per mile.