Tax code defaulted in the PO

Hi Gurus,

Recently there is a issue. While creating PO, a default tax code I1 is retrieved by the system automatically . Tax amount is come to SAP  from third party system through RFC. But, our requirement is if tax amount=0, then I0 is the tax code, if tax amount > 0, then I1 is the tax code. there is functional module BBP_CALCULATE_TAX_ITEM46B implemented for tax functionality. How to read the functional module BBP_CALCULATE_TAX_ITEM46B , I have no idea on Functional module. Your idea will be appreciated.

Best regards,

Nagesh