Professional liability covers claims that your professional judgement was wrong: a design that did not work, a specification that was incorrect, advice that cost the customer money.
It is often written as errors and omissions, or E&O. In healthcare it is usually called malpractice.
The distinction that matters
General liability covers physical harm — you damaged something or hurt someone.
Professional liability covers financial harm from your expertise. Nothing broke; the customer lost money because you were wrong.
A designer specifying the wrong system, a consultant giving advice that fails, a practitioner making a clinical error. General liability does not reach any of it.
Who needs it
Anyone paid for judgement rather than only for labour: consultants, designers, architects, engineers, medical and allied health, accountants, agencies, and increasingly contractors who do design-build.
A pure install-to-specification trade may not need it. A trade that also advises what to install probably does, and the line moves as a business grows into advising.
Claims-made cover
Most professional liability is written on a claims-made basis, which behaves differently from other policies: it covers claims made while the policy is active, not incidents that happened while it was active.
Letting it lapse can therefore leave past work uncovered. If you stop trading or switch insurer, ask about run-off or tail cover, which extends the window for claims arising from work already done.
Where it appears
Insurance and compliance in your accounts, and a fixed cost for break-even purposes.