If you took the special depreciation allowance the first year you placed an item in service, enter the amount of special or "bonus" depreciation taken in the Prior Special Depreciation field. This amount is shown on the previous year's tax return (Depreciation Statement in the Bonus Depreciation column).
Special depreciation is an extra allowance that you can take the first year a property depreciated under the MACRS method is placed in service. This additional allowance is automatically calculated unless you check the opt-out of special depreciation check box. For most assets placed in service after January 1, 2018, the Special Depreciation is 100% of the cost based on the new tax law.