Good morning victor,
WELCOME TO THE AWS WELDING FORUM!!
D1.3 is an AWS established code that is titled 'Structural Welding Code-sheet steel. Is is used on materials of thin gauge up to 3/16".
It is used on projects from decking attached to joists and girders, metal studs, and many other such items.
On larger building projects you may have D1.1 on the main structural steel, to D1.4 for rebar, and D1.3 for the light gauge steels on floors and roofing.
Hope this answers your question.
He Is In Control, Have a Great Day, Brent