If I may add some confusion, with regard to structural applications, “HSS” (Hollow Structural Sections) is the correct designation for structural tubing, rather than “TS” (tube shape). In 1990, the industry adopted the term HSS to keep in step with producers around the world. In 1997, the Steel Tube Institute and AISC officially implemented this change with the publication of the Specification for the Design of Steel Hollow Structural Sections and the corresponding AISC/STI/AISI HSS Connections Manual. HSS are also available in round, square, or rectangular shapes. Round shapes are specified using decimal numbers and three decimal points. For example, HSS 5.563 × 0.258 indicates a round HSS with an outside diameter of 5.563” and a wall thickness of 0.258”. Rectangular and square shapes are similarly specified but in terms of rational numbers. For example, HSS 5 × 4 × 3/8 indicates a rectangular HSS with a depth of 5”, a width of 4”, and a wall thickness of 3/8”.
For rectangular, square, and round,HSS, the most common material specification is ASTM A500 Grade B (46 ksi yield and 58 ksi tensile). Although HSS rounds are sometimes used in structural applications, most of the pipe used in structural applications is ASTM A53 Grade B (35 ksi yield and 60 ksi tensile). ASTM A53 is produced in Schedule 40 (standard, Schedule 80 (extra strong), and Schedule 120 (double extra strong). A53 pipes are specified using a nominal inside diameter. For example, a 4” Schedule 40 (standard pipe) is 4.03” inside diameter, 4.50 outside diameter, with a nominal wall thickness of 0.237”. A 4” Schedule 80 (extra strong pipe) is 3.83” inside diameter, 4.50 outside diameter, with a nominal wall thickness of 0.337”. A 4” Schedule 120 (double extra strong pipe) is 3.15” inside diameter, 4.50 outside diameter, with a nominal wall thickness of 0.674”.
A piping system is when a collection of the products/ assemblies from "Spool" or isometric drawings are connected together by either welding them, and/or some of these spools are connected together via flanges that may have various types of gasket materials which cause a seal when tightened together to prescribed torques, and sometimes there are various types of fittings, valves, pumps, bellows, catch drains, sight glasses, and/or or other instruments that are connected in between these piping spools which when connected all together are sometimes called "runs' by some in the know also... Some of these spools also may have branched fittings welded, and then connected further off the run via flanges too... 
Is this a trick question?????
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