Congress created a special tax credit as a way of refunding the federal excise tax paid on gasoline and diesel fuel to certain business users - particularly to farmers and commercial fishermen who use the fuel in their businesses. In the Interview, you'll find other uses that qualify for this credit. The per-gallon credit rate varies depending on the type of fuel involved.
Form 4136 is used to claim the credit, but if a deduction including the tax was taken on Schedule C , the amount of the credit has to be included in Gross Income.
See IRS Pub 378 for more information on Fuel Tax Credits and Refunds.