Using VNEXR1 on F0911Z1

We already load batch invoices via F03B11Z1/F0911Z1.

We need an indicator on some of the invoice line items that happen to be taxable items (not all items on an invoice are taxable).

Was thinking to use VNEXR1 and populated a value of "T" on it and also defined a UDC as 00/EX for "T" taxable but the value doesn't get populated on the F0911 table.

Not sure why.

Any ideas and/or ideas of a 1 character field we can use on F0911Z1 to indicate taxable that will flow thru to the F0911 table?

Thanks,

Bruce Ray