A mid rail is an internal horizontal member of a gate frame, added between the top and bottom rails. It does two jobs at once. It stiffens the frame against in-plane racking. And it gives the middle of long battens or slats a fixing target so they don't bow under wind load.
Mid rails are optional on shorter gates (anything under about 1.5 m tall typically gets by with just top and bottom rails). They become standard on taller gates, double-swings with longer battens, and any gate carrying a mounted access controller or intercom that needs a steel substrate behind the bracket.
CAD60 surfaces mid-rail count as a per-model input. Some models default to 0; others (like very tall sliding gates) default to 1 or 2.