Dear Ahemmed,
Thanks for the reply. There is a new document type created for the reversal of invoice as in the standard document type AB is assigned to RV as the reverse document type.
And USEREXIT_NUMBER_RANGE USING US_RANGE_INTERN (INTERNAL NUMBER RANGE) is used to assign number based on the Sales organization (VKORG), Plant (WERKS) and billing document type (FKART).
Thanks,
Andy