Public liability and professional indemnity are told apart by asking what was lost. Physical harm or damaged property is public liability. Money alone is professional indemnity. The interesting cases are the ones where both are true at once, and those are why holding them with one insurer is worth something.
Clearly public liability
A client trips over a trailing cable in your workspace. A tool falls and dents a car. A pipe you fitted leaks into the flat below. Somebody is hurt or something is broken, and the claim is for that harm.
Clearly professional indemnity
A report with an error the client acted on. A design that did not meet the brief. A tax return that produced a penalty. Advice that led to a contract being lost. Nothing is damaged and the client is out of pocket.
The cases that touch both
A negligently designed structure that then fails and injures somebody. A specification error that caused a flood. Here the design failure is professional indemnity and the injury or damage is public liability, and a single incident produces two claims. One insurer holding both is what stops the two policies pointing at each other while the claimant waits.
Which one a client is asking for
Read the requirement rather than the phrase. A contract naming a figure for injury and damage means public liability; one naming a figure for professional services or advice means professional indemnity. Where a client asks for indemnity cover without saying which, the fastest resolution is to ask what limit they require for each, because the answer settles it in one exchange.
Questions people ask about public liability vs professional indemnity
How do I know which applies?
Ask what was lost. Physical injury or property damage is public liability; money alone is professional indemnity.
Can one incident trigger both?
Yes, where a design or advice failure also caused injury or damage. Holding both with one insurer avoids an argument between them.
Which is more expensive?
It depends entirely on the profession. For advisory businesses professional indemnity usually costs more; for site based trades public liability often does.