RHS is one of two dominant gate-frame steel sections (the other is square hollow section, SHS). It's made to AS/NZS 1163 in grade C350 (350 MPa yield) for general structural use. Common gate-frame sizes are 50x25, 65x35, 75x50, and 100x50 mm with wall thicknesses of 1.6, 2.0, 2.5, or 3.0 mm.
RHS goes on rails and stiles where the deeper dimension matters for bending. A 65x35 RHS used long-side-vertical resists sag in a 1.2 m gate stile better than a 50x50 SHS at the same wall thickness, for similar steel cost.
CAD60 model frames default to industry-standard RHS sizes. Change the profile and the gate, hinge plates, and dimension lines all recompute.