Dealing with Withholding tax on an interface file coming into Oracle

Has anyone any experience of how to deal with withholding tax when invoices liable to CIS come into Oracle via an interface please?

I was hoping that we could flag the supplier as a sub-contractor, then identify the labour line on the file from the feeder system, so that the Withholding tax code is retained on that line in Oracle. The materials line would come down without a flag so the Withholding tax code is  erased on that line, but am having difficulty in obtaining information on how this could be done.  That way the CIS deduction would be calculated in Oracle.