A bracing bar is a horizontal steel tube (typically a 25x25 or 30x30 SHS) welded across the back of a gate's battens or slats. It sits behind the infill so it's invisible from the gate face but adds significant lateral stiffness, stopping individual battens or slats bowing or rattling under wind load.
Bracing bars are common on tall gates, slatted gates with vertical slats, and any gate where the infill spans more than about 1.8 m unsupported.
CAD60 surfaces a per-model 'bracing bar' yes/no toggle, plus a 'bracing bar height' control that places the bar at any height between top and bottom rails. The drawing pack shows it on the front and side elevations.