Per IRS Publication 946 How to Depreciate Property, starting on page 15:

You can elect to recover all or part of the cost of certain qualifying property, up to a limit, by deducting it in the year you place the property in service. This is the section 179 deduction. You can elect the section 179 deduction instead of recovering the cost by taking depreciation deductions.

On page 16:

Eligible Property

To qualify for the section 179 deduction, your property must be one of the following types of depreciable property.

1. Tangible personal property.
2. Other tangible property (except buildings and their structural components) used as:
   a. An integral part of manufacturing, production, or extraction, or of furnishing transportation, communications, electricity, gas, water, or sewage disposal services;
   b. A research facility used in connection with any of the activities in (a) above; or
   c. A facility used in connection with any of the activities in (a) for the bulk storage of fungible commodities.
3. Single-purpose agricultural (livestock) or horticultural structures. See chapter 7 of Pub. 225 for definitions and information regarding the use requirements that apply to these structures.
4. Storage facilities (except buildings and their structural components) used in connection with distributing petroleum or any primary product of petroleum.
5. Off-the-shelf computer software.
6. Qualified section 179 real property (described below).

Please see Chapter 2 of IRS Publication 946 for a detailed description of these properties.

Property Acquired for Business Use

To qualify for the section 179 deduction, your property must have been acquired for use in your trade or business. Property you acquire only for the production of income, such as investment property, rental property (if renting property is not your trade or business), and property that produces royalties, does not qualify.

Partial business use. When you use property for both business and nonbusiness purposes, you can elect the section 179 deduction only if you use the property more than 50% for business in the year you place it in service. If you use the property more than 50% for business, multiply the cost of the property by the percentage of business use. Use the resulting business cost to figure your section 179 deduction.

Starting on page 17:

How Much Can You Deduct?

Your section 179 deduction is generally the cost of the qualifying property. However, the total amount you can elect to deduct under section 179 is subject to a dollar limit and a business income limit. These limits apply to each taxpayer, not to each business. However, see Married Individuals under Dollar Limits, later. For a passenger automobile, the total section 179 deduction and depreciation deduction are limited. See Do the Passenger Automobile Limits Apply in chapter 5.

If you deduct only part of the cost of qualifying property as a section 179 deduction, you can generally depreciate the cost you do not deduct.

On page 18:

Dollar Limits

The total amount you can elect to deduct under section 179 for most property placed in service in tax years beginning in 2020 generally cannot be more than $1,040,000. If you acquire and place in service more than one item of qualifying property during the year, you can allocate the section 179 deduction among the items in any way, as long as the total deduction is not more than $1,040,000. You do not have to claim the full $1,040,000.

TIP: The amount you can elect to deduct is not affected if you place qualifying property in service in a short tax year or if you place qualifying property in service for only a part of a 12-month tax year.

CAUTION! After you apply the dollar limit to determine a tentative deduction, you must apply the business income limit (described later) to determine your actual section 179 deduction.

Please refer to the publication for additional details about the section 179 deduction.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.