Hi,
If the sales order is having tax details then in VF01 ,you have to choose Tax Billing Type not then choose Retail Invoice Billing Type ( maintain two different billing number ranges in billing types and say to ABAPer to them into print out for identification ).
Client would have provided their existing printouts of these.So,take that printouts and ask ABAPer to make them in SAP.By this way,you can process.
Thanks,
Naren