This is totally unofficial, but this is what I found and where I found it:
(d) Qualified trade or business
For purposes of this section-
(1) In general
The term "qualified trade or business" means any trade or business other than-
(A) a specified service trade or business, or
(B) the trade or business of performing services as an employee.
(2) Specified service trade or business
The term "specified service trade or business" means any trade or business-
(A) which is described in section 1202(e)(3)(A) (applied without regard to the words "engineering, architecture,") or which would be so described if the term "employees or owners" were substituted for "employees" therein, or
(B) which involves the performance of services that consist of investing and investment management, trading, or dealing in securities (as defined in section 475(c)(2)), partnership interests, or commodities (as defined in section 475(e)(2)).
https://uscode.house.gov/view.xhtml?req=%28title:26%20section:199A%20edition:prelim%29
*Section 1202(e)(3)(A)* says...
(3)Qualified trade or business For purposes of this subsection, the term “qualified trade or business” means any trade or business other than—
(A)
any trade or business involving the performance of services in the fields of health, law, engineering, architecture, accounting, actuarial science, performing arts, consulting, athletics, financial services, brokerage services, or any trade or business where the principal asset of such trade or business is the reputation or skill of 1 or more of its employees,
(B) any banking, insurance, financing, leasing, investing, or similar business,
(C) any farming business (including the business of raising or harvesting trees),
(D) any business involving the production or extraction of products of a character with respect to which a deduction is allowable under section 613 or 613A, and
(E) any business of operating a hotel, motel, restaurant, or similar business.
Found at: https://www.law.cornell.edu/uscode/text/26/1202
