What the lump sum really is
A settlement offer is stated as a share of the balance: settling $15,000 of debt at 40% means paying $6,000 in one lump sum, and the other $9,000 is forgiven - the portion you no longer owe. The page keeps the deal simple by asking for the offer directly rather than simulating years of negotiation, because a settlement is only real once a creditor actually accepts it.
That forgiven share is where the tax wrinkle lives, and it is easy to underestimate. The $9,000 written off is not simply savings you get to keep; in the eyes of the tax code it may be income, which is exactly why the page separates the lump sum from the tax rather than showing a single rosy number.