Swearingen is right. The "old" green steel book has TS sections and pipes. The new black book has HSS sections, and round hollow sections (pipes) also noted as being HSS sections. There are some minor differences in wall thickness between SOME TS and HSS section, suggesting a tightening of tolerances or maybe even an attempt a "metrification". Please note the round HSS sections. It suggests that the pipes will be going away too.
To my understanding, the HSS shapes were devised because most mills were creating TS shapes at the low end of the tolerance for the wall thickness, thus creating shapes with smaller section property values than what was given by the nominal wall thickness. The HSS designation uses a reduced wall thickness to calculate the section properties. If I remember correctly, the design wall thickness is 92% of the nomimal wall thickness called for in the shape designation.
