To create a tax-only line, you have to create a dummy line and
link your tax line to the dummy line.

Create a dummy line of type 'LINE' in RA_INTERFACE_LINES table
           interface_line_context    = 'Order Entry'
           interface_line_attribute1 = trx_number
           interface_line_attribute2 = 1
           interface_line_attribute1 = 'LINE'

and columns amount, quantity, quantity_ordered,
unit_selling_price, unit_standard_price should be set to zero.

Create a line of type 'TAX' and link this line to the dummy line
           interface_line_context= 'Order Entry'
           interface_line_attribute1 = trx_number
           interface_line_attribute2 = 1
           interface_line_attribute1 = 'TAX'
           amount = tax amount

Now you need to link the TAX line to the Dummy Line Created
above and for that you need to set link_to_line_attribute.

           link_to_line_context    = 'Order Entry'
           link_to_line_attribute1 = trx_number
           link_to_line_attribute2 = 1
           link_to_line_attribute3 = 'LINE'

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