Commercial general liability (CGL) policies for contractors traditionally contain a professional liabilities exclusion. This exclusion is generally added through a specific endorsement to eliminate coverage for professional services. Read the endorsement The point of the exclusion, in a nutshell, is simply to eliminate a CGL policy for a contractor serving as a professional liability policy.
Contractors need to appreciate a professional liabilities exclusion added through endorsement because oftentimes there are delegated design components they are responsible for. Perhaps the contractor value engineered a system and is responsible for engineering and signing and sealing the engineered documents (through its subcontractor) associated with that system. Perhaps there is a performance specification that requires the contractor to engineer a system. Perhaps there is a design-build component. Regardless of the circumstance, this professional liabilities exclusion can certainly come into play, particularly if a defect is raised with the design or professional services associated with the engineered system.