The execution class is chosen from the consequence of failure and the type of loading, and it drives everything downstream: welder and procedure qualification requirements, the extent of visual and volumetric NDT, and the depth of the documentation.
EN 1090-2 ties the required welding quality level (ISO 3834 part) to the class — EXC1 to 3834-4, EXC2 to 3834-3, EXC3 and EXC4 to 3834-2.
The recurring mistake
Deciding the NDT extent job by job from memory. As the class rises the required coverage rises with it, and hand-picking the percentage is where inspection quietly drifts below what the class demands — a finding that should never have been a finding.