Considering flexure on rectangular or square Hollow Structural Steels, the section must be checked on its yielding, flange local buckling and web local buckling to determine its normal moment capacity. The lowest nominal capacity to be considered shall be the lowest of the three conditions. To get the design flexural strength of a section whether HSS or not, the nominal capacity would be multiplied by 0.90 when considering LRFD. On the other hand, a divisor of 1.67 shall be used when considering ASD. The 0.90 and 1.67 are the omega values for LRFD and ASD respectively.
I hope I answered your query.