A side-mount post bolts to the vertical face of a concrete slab edge, masonry wall, or steel beam, instead of standing on top of the slab. The post sits below the slab top, with a heavy fabricated bracket that wraps around the slab edge.
Side-mount keeps the slab top fully clear, which suits pool surrounds where the deck is cut to a tile line, or balconies where the floor finish runs right to the edge. It also lowers the post tops by the slab thickness, which can help line them up with adjacent structure.
Downside: the bracket is a stress concentrator and the bolts are working in tension instead of compression. Side-mount fixings need careful detailing and usually a thicker bracket plate than a top-mount footplate would require.