The Limit States Design specification allows inelastic deformation of section elements. Thus local buckling becomes an important criterion. Steel sections are classified as compact (type 2), noncompact (type 3),  or slender element (type 4),  sections depending upon their local buckling characteristics, besides sections type 1 are able for plastic design. This classification is a function of the geometric properties of the section. The design procedures are different depending on the section class. STAAD is capable of determining the section classification for the standard shapes  and design accordingly.