RM4190 | RO instructions: When to use the `AP' procedure
From HM Revenue & Customs · Repayment Claims Manual
AP' stands for after payment. Use the AP’ procedure when you have no reason to doubt that the vouchers are valid and the voucher is one of the following
an R185 series form other than one showing
trust income
income which arose during an administration period
for the first time an address outside the UK for the payer
a typed or handwritten dividend warrant counterfoil from a UK company or local authority.