Why Diode symbols in the figure all share the same triangle-pointing-at-a-bar shape, and the shape of the bar tells you the type. The Schottky diode uses a cathode bar bent into a squared-off S at both ends, matching symbol 6. A Schottky is a metal-to-semiconductor junction rather than a PN junction, which gives it a low forward drop (about 0.3 V) and very fast switching, so it shows up in RF detectors and switching supplies.