A three—digit code used to identify any supplementary conditions that apply to the intended use of the goods. For example, if customs relief for both Customs Duty and Import VAT are being claimed, two separate Additional Procedure Codes (APC) will need to be quoted. One is required to claim Customs Duty relief, the other for Import VAT relief. The Customs Declaration Service (CDS) allows up to 99 Additional Procedure Codes to be to be used with a single four—digit Procedure Code.
For further information, please refer to GOV UK.