The following example uses a MERGEREC field inside an = (Formula) field to create unique invoice numbers. When the main document is merged with the data source, the number resulting from the MERGEREC field is added to the numbers representing the date and time the invoices are printed.
Field:
Invoice Number: { = { PRINTDATE \@ "MMddyyyyHHmm" } + { MERGEREC } }
Result:
Invoice Number for record 12, printed on Feb. 13, 2003 at 9:46:
02132003094612