Some sample reasons that make Welder Certification, WPS and WPQR documents mandatory and necessary:

Pressure Equipment Directive (PED), EN 12952, EN 12953, CE Certification, EN 303-1-3, EN 3834, EN 1090-1, EN 1090-2, EN 15085-2, Tanker, Trailer Design Approval Instruction, TS EN Can be listed as 10217-2, EN 1090-1 and EN 1090-2.