This is what I found on the IRS website.  Is there a better reference? One that would hold up in court....

A5. A qualified trade or business is any trade or business, with two exceptions:

1. Specified service trade or business (SSTB), which includes a trade or business involving the performance of services in the fields of health, law, accounting, actuarial science, performing arts, consulting, athletics, financial services, investing and investment management, trading, dealing in certain assets or any trade or business where the principal asset is the reputation or skill of one or more of its employees. This exception only applies if a taxpayer’s taxable income exceeds $315,000 for a married couple filing a joint return, or $157,500 for all other taxpayers
    
2. Performing services as an employees

These FAQs are not included in the Internal Revenue Bulletin, and therefore may not be relied upon as legal authority. This means that the information cannot be used to support a legal argument in a court case.