A notice of coding is issued to an individual where there is a variation in their tax code away from the 'base' code generated by the standard UK personal allowance in force. The notice includes the calculation for each item that changes the code and then a specific note for each item in explanation of the change. The purpose of the code change, as detailed on the notice, is to ensure that, as best possible, the correct amount of tax is collected over the course of the tax year.