AGIS question: cannot find VAT Taxable flag in IC Transaction Type window

the table where the IC Transaction Type data is stored does have a VAT Taxable Flag, and this field is also listed as one of the Sources in the AGIS Subledger Accounting Set up pages. However in the AGIS set up screens, nor in the Outbound batch can I influence the VAT behaviour.

How can I best influence whether tax should be added (and which tax rate), in AGIS itself, rather than assigning it at customer / supplier site level, and let the AR and AP transaction interface handle the tax calculation?

Regards
Reina