The regulation answers many of those questions, though some remain up for interpretation. Owners of dental offices may not claim the deduction, it says, but owners of health spas may, because they do not directly provide medical services. Investment banks do not qualify, but banks that simply make loans and take deposits — like community banks — do. Paralegals may not, but the owner of a stenography service may. Football coaches do not qualify, but the owner of a company that cleans football stadiums would.

The regulation defines the “reputation or skill” exclusion narrowly. It does not appear to apply to writers, which means some authors with lots of book income could organize themselves as pass-through companies and qualify for the deduction. It does apply to celebrities who license their names or voices, endorse products or accept appearance fees, “including fees or income to reality performers performing as themselves on television, social media or other forums.”

That provision appears to allow a company that earns some of its income by licensing its owner’s name to apply the 20 percent deduction to other income that does not flow from licensing. That could be important for Mr. Trump, whose companies have licensed the Trump name to hotels and products around the world. Tax lawyers said Wednesday that the regulations could have prohibited Trump companies from deducting much more — if not all — of their income, instead of allowing the income streams to be separated.

“To the extent that he’s getting income from licensing the use of his name, it looks like he is ineligible for the deduction,” said Lily Batchelder, a New York University law professor and former Obama administration official. “But even if that licensing income was, hypothetically, 51 percent of his business’s income, I think it’s treated as a separate trade or business so he can get the deduction on the other 49 percent of his income.”

Ms. Batchelder said the narrow reading of the “reputation or skill” provision would allow tax breaks for wealthy owners of advertising agencies and other pass-through entities that trade largely on the abilities of their founders.

The regulation helps to resolve questions about whether some companies might need to restructure to comply with the law — or whether they would be allowed to restructure to game the rules. It outlaws a strategy known as “crack and pack,” where business owners split their operations into separate entities — for example, one that owns a doctor’s office and another that provides medical care — to avoid prohibitions on taking the deduction.

It also allows businesses to aggregate the activities of a group of related companies to maximize tax benefits in several ways. Some organizations, for example, pay employees through a different entity than the one that brings in revenue. Business groups told Treasury officials this year that they worried those organizations would not be allowed to claim a full deduction, because they would not appear to pay any employees. The regulation soothes that fear, in a way “favorable to taxpayers and not provided for in the statute,” said Howard Wagner, national tax services partner at Crowe LLP.