The common denominator: EN 1090-2 requires it at every execution class, pressure work adopts it as the welding quality system, and ASME IX is its counterpart outside Europe.

Why this one comes first

Almost every other welding regime leans on ISO 3834.

EN 1090-2 references it for all structural work, the level rising with the execution class (EXC1 → 3834-4, EXC2 → 3834-3, EXC3/4 → 3834-2).
Pressure equipment makers use it as the welding quality system behind PED.
Outside Europe, ASME Section IX plays the qualification role and the ASME BPVC QC/QA programme behind the stamp the quality-system role.

That is why the product has one backbone and not four: the evidence a welding shop must produce is largely the same, and only the regime on top changes.

The three levels

The standard comes in comprehensive, standard and elementary requirements. The level is not a grade of quality — it is a match to the work being done. Choosing a level higher than the work needs buys audit burden, not confidence.

What it asks you to be able to show

Which procedure covered this joint, in the revision that was current when it was welded
Which welder made it, qualified in a range that actually contains this joint
That the consumables and parent material were the ones specified, traceable to a heat
That inspection happened at the extent required, by someone qualified to perform it
That deviations were dispositioned, not quietly absorbed