The Sub Default is Zero. You should change this only if you have a department within a department (that has different characteristics) and you need a separate key on the Checkstand keypad.  For example, Grocery and Grocery taxable both have a separate key on the keypad, but both sales go into the Grocery Department. So when an item comes through the checkstand without a UPC code, the amount of the item can be rung up under Grocery or Grocery tax as appropriate, but the Grocery Department gets the credit for the sale.