Refund auto numbering?

Good afternoon,

We are using Issue Refund function against Credit memos or Payments in Oracle AR (table=AR_RECEIVABLE_APPLICATIONS_ALL >>a.APPLICATION_REF_TYPE='AP_REFUND_REQUEST' )  and these refunds then flow onto the AP side where they are called invoices (table=AP_INVOICES_INTERFACE >>where PAYMENT_FUNCTION='AR_CUSTOMER_REFUNDS') .

We have noticed that the numbering is off, there is no rhyme and reason to it and I would like to know where is this configurable please - where does the number of the refund come from when you first create it, please? I am assuming it's somewhere under Setup & Maintenance but couldn't find the exact please.