Field help (SIHTYP) 

Invoice type (Invoice/Credit note/Pro forma)

It determines the document type as well as the sequence number counter used. It is only accessible during creation.

The choice of Invoice or Pro forma automatically implies the creation of an invoice or direct pro forma.

The Credit note choice by default the creation of a direct credit note.

To generate a credit note on an invoice, it is necessary to enter the original document number with the corresponding invoice number.