Some shaped saddle pads are made of gel material to provide cushioning.
A shaped (or "numnah") saddle pad is saddle shaped, and is designed to be used alone, or with a square saddle pad. A shaped / numnah pad should only be slightly visible below the saddle.
Gels are gelatin-like polymer materials that are solids, but which absorb shock and dissipate energy like liquids. The drawback to gels is that they are quite heavy, and expensive.