It goes on the feed tube for the cam oil shower pipes. It's the thing that sticks down from the valve cover on the drivers side (this is why you pull straight up on that side when removing the cover).most importantly, where does that o-ring go? - not shown in the pictures.
The four larger gaskets are the spark plug tube oil seals. They can be reused, though I would consider them a normal service item and replace them periodically, else they'll leak and you'll end up with plug tubes full of oil, and water contamination.