I have discoverd a field on the Edit>QBO_ItemService screen (I copied it to edit) that is a dropdown with a label "Tax Code". However it doesn't show up on the actual data entry/edit screen. I cannot see anywhere that the field is hidden at either the object level or the page load level. It is referencing the field "SalesTaxCode". I suspect it's not showing on the data entry/edit screen because it doesn't appear that field is in the ItemService table.
I had considered adding it, but did not for fear of creating conflicts elsewhere or in the QBO sync. I suspect there are other formulas in sales orders and invoices that are or should be looking at this field.