On a framed metal gate the hinges bolt or weld to a vertical hinge stile running the full height of the leaf. A timber ledged-and-braced gate has no perimeter frame and no stiles at all: the same two or three horizontal rails the pickets are face-fixed to also carry the hinges, one bolt-on plate hinge centred on each rail. That rail is then doing the job a stile would do on a welded gate, which is why it is called a ledge rather than just a rail once the leaf is hung on it.
A ledged gate keeps the fence family's datum, ground at the picket foot rather than a frame bottom at zero, so the ledges sit inset from the top and bottom of the leaf by the same rail-inset measurements a flat fence panel uses. Because the ledges already carry their own dimensioned insets, the hinge heights are stated as "one hinge per ledge" in the drawing note rather than as a second stack of height dimensions running down beside them.
Ledged-and-braced construction is the traditional way a timber gate is built: no mortise and tenon, no welded frame, just ledges, a diagonal brace, and pickets, all screwed or nailed together. It is lighter and faster to build than a framed gate and is the standard method behind every CAD60 timber picket swing gate.