Many commercial policies, especially surplus lines contractor and small business forms, include an endorsement that limits coverage to the operations described on the declarations or the application. If the client does work that isn't described, claims from that work may not be covered at all.
This is how side work gets clients into trouble. A painter who also does small electrical jobs, or a handyman who starts taking roofing repairs, may have no coverage for those jobs even though the policy is in force.
The short version
The business description on the dec page isn't a label. On many policies it's the edge of the coverage. Ask what else the client does, and get it on the policy or on a separate one.