What is a "specified" service business?

[CarolineM]

Specified service businesses includes occupations that provide personal services, except for engineering and architecture.

Examples of “specified” service businesses include:

• Healthcare services performed by doctors, nurses, and dentists
• Law, accounting, and actuarial services (including anyone preparing taxes)
• Performing arts
• Consulting
• Athletics

[Carl]

With the exceptions you noted, if you have to go out and actually "do something" on a recurring basis to make money, that's a specified service business.
So for example, if you are a landlord that owns rental property, that's not a specified service business. You don't "do anything" on a recurring basis to make that monthly rental income. All you do is "sit there" and collect the rent when it's due.
But when it comes to QBI, rental property is somewhat of a grey area still. Basically, if you can "prove" you spent more than 250 hours directly involved in the rental property, then the income can qualify for QBI. But things like the bookkeeping & rent collection do not count as being "directly involved" in the rental property.