The hinge stile is the side of a gate that bolts or welds to the hinges. It carries the entire gate weight in shear and the entire wind moment in bending, so it's the highest-stressed member in the gate frame.
For heavy gates, the hinge stile is often upsized over the latch stile. A typical pattern: 65x35 RHS for the latch stile, 65x35x2.5 (instead of 2.0 mm wall) for the hinge stile, with internal stiffener plates welded behind each hinge mount. CAD60 surfaces this as one frame-tube setting that applies to both stiles by default, with optional hinge-stile-only reinforcement.
Placed wrong on installation, the hinge stile pulls the gate out of plumb within months. Get the hinge centres exactly vertical at install or the gate will sag toward the ground at the leading edge.