It definitely existed. As explained in this Guide:
- Choose 100% to create a tax code for line items that are all tax… Forms will show the tax amount as a regular line item rather than an addition to the subtotal….
In other words, the built-in 100% tax code adds a line item with Unit price as the full Amount. That amount appears in the subtotal. In the following illustration, assume you have purchased 100.00 worth of goods, taxable at 10%, but the supplier rounds differently (ignore the fact this could not happen in this simple example) and invoices you for 115.00. You enter the purchase invoice with a tax adjustment line using the built-in custom tax code rate of 100%. Your purchase invoice matches the supplier’s sales invoice:
In the second illustration below, a custom tax code defined as Custom => Single rate => 100%, as illustrated by @brucanna, is used instead. This taxes the Amount at 100% and adds that tax to the subtotal. Now your purchase invoice does not match the supplier’s sales invoice:
Which approach to use depends on what you are trying to match.