Almost always for connections. Flat faces allow square cuts at intersections, let plates and brackets seat directly, and accept fixings, clamps and panels without shaped packers. On a frame with many joints that saves substantial fabrication and installation time. Structurally the two are comparable, both having equal properties about their principal axes, so the choice is usually driven by detailing rather than by loading.

Equipment frames and skids, machine guarding, balustrade and glazing frames, shopfitting and display, furniture, and structural framing in food, beverage and pharmaceutical plant where the frame must be washed down. Anywhere that combines a need for corrosion resistance or cleanability with a lot of connections and panel or glass attachment.

On exposed and hygienic work, yes. Every hollow section has a formed radius at its external corners, and its size varies with wall thickness and between manufacturers. Where sections are mitred or butted flush against one another, mismatched radii are visible. In hygienic applications the internal corner is the hardest area to clean, which occasionally favours round sections despite the connection penalty.

Square sections have the same stiffness and capacity about both axes, so they suit members loaded in more than one direction, columns and general framing. Rectangular sections concentrate their capacity about the stronger axis and suit members bending predominantly in one plane, such as beams and rails. Using a square section where a rectangular one would serve usually costs weight and money.

Work hardening and heat. Cutting and drilling need sharp tooling, firm feed and slower speeds, since dwelling hardens the surface and the next pass then struggles. Welding needs matched consumables and post-weld cleaning, because heat tint reduces corrosion resistance if left. Tools, abrasives and brushes previously used on carbon steel must never be used, as embedded iron causes rust staining that looks like a material failure.

Austenitic grades such as 304 and 316 are essentially non-magnetic when annealed, but become slightly magnetic where they have been cold worked, such as at formed corners and cut edges. That is normal and is not an indication of the wrong grade. Duplex grades are magnetic by nature. Magnetism is therefore not a reliable field test for distinguishing stainless grades.

Mill finish for concealed work, brushed or satin finishes for most exposed applications, and polished or mirror finishes for architectural and display work. Finish should be specified together with the weld seam condition, and it is worth confirming that all sections for one installation come from the same batch, since brushed finishes vary noticeably between production runs when placed side by side.