The way you add (sales) tax to items is by checking the "Apply tax" checkbox for the item, in
Invoice processing.
The "taxed" column in product databases is not currently used anywhere. If you are curious about what/where/how a column is used, this is the authoritative document to check:
http://foodclub.org/wiki/Foodclub:Docum ... ase_fieldsThe "taxed" column
should probably be used as the default state of the "Apply tax" checkboxes, on the first load of
Invoice processing before invoices have been generated. But since nobody has ever requested it, it hasn't been done and the taxed column is just sitting there unused.