AWS D1.1 is the American Welding Society's Structural Welding Code--Steel. D1.3 is Structural Welding Code--Sheet Steel. D1.1 covers the fabrication and erection of welded structures using mild or carbon steel 1/8" thick or greater. D1.3 covers thinner material. They both cover the requirements for design of welded connections, welding procedures (prequalified procedures in the code and qualification of procedures by the fabricator), qualification of personnel (for different processes, positions, material thicknesses, etc.), fabrication, weld inspection (visual, ultrasonic, radiographic, etc.) and repair.
These codes are similar to model codes like the National Electrical Code in that they define acceptable practice for an industry, may have the force of law if include by reference in the laws or regulations of a government entity, may be contractual requirements if included by reference in a contract or suporting documents such as a drawing, etc.
When adherence to these codes is required, one or more Certified Welding Inspectors may be involved, possibly for both the fabricator and the company having the work done.
Don't suppose this was just something put on an internal print that doesn't really need it?