Per IRS Publication 946 How to Depreciate Property, starting on page 23:
You can take a special depreciation allowance to recover part of the cost of qualified property (defined next), placed in service during the tax year. The allowance applies only for the first year you place the property in service. The allowance is an additional deduction you can take after any section 179 deduction and before you figure regular depreciation under MACRS for the year you place the property in service.
What is Qualified Property?
Your property is qualified property if it is one of the following.
On page 26:
How Much Can You Deduct?
Figure the special depreciation allowance by multiplying the depreciable basis of qualified reuse and recycling property, certain qualified property acquired before September 28, 2017, certain qualified property acquired after September 27, 2017, and certain plants bearing fruits and nuts by the applicable percentage.
For qualified property other than listed property, enter the special depreciation allowance on Form 4562, Part II, line 14. For qualified property that is listed property, enter the special depreciation allowance on Form 4562, Part V, line 25.
TIP: If you place qualified property in service in a short tax year, you can take the full amount of a special depreciation allowance.
Depreciating the remaining cost. After you figure your special depreciation allowance for your qualified property, you can use the remaining cost to figure your regular MACRS depreciation deduction (discussed in chapter 4). Therefore, you must reduce the depreciable basis of the property by the special depreciation allowance before figuring your regular MACRS depreciation deduction.
Please refer to the publication for additional details about the special depreciation allowance and the section 179 deduction.
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