Rather than me trying to explain it all, take a look at the commentary section, C26, on page 408. Basically, the design criteria for box sections doesn't include provisions for materials that are within the minimum yield strength range of over 52 ksi - 75 ksi (Group III) because further investigations are needed with regard to their weldability, fatigue problems, and notch toughness issues. That's not to say they can't be used. They just require more considerations. Steels in Groups IV and V (over 75 ksi - 100 ksi) require other considerations, and the code is not intended for use with materials with a minimum yield strength of over 100 ksi.