Message SAP VT621 - The assignment and reference number fields must be ' ' or E

Diagnosis In the controls for copying billing document (F2) from credit memo (CR)or debit memo (DR) the assignment number and reference number fieldsshould be ' ' or contain 'E'. This message appears if VBTYP for the source document is 'M' and VBTYPfor the target billing document is '0' or 'P'. This copy control meansthat the billing document is created directly from a credit memo or adebit memo. Procedure Leave the assignment number and reference number field blank or enter'E'.