The qualified business income deduction applies only to a trade or business. Under an IRS-created safe harbor, a rental real estate activity may be treated as a trade or business for purposes of this deduction. Details of this safe harbor are in IRS Revenue Procedure 2019-38 (https://www.irs.gov/pub/irs-drop/rp-19-38.pdf).
The IRS action of fixing tax liability that sets in motion collection procedures, such as charging interest, imposing penalties, and, if necessary, seizing property.