The retaining nut that secures an AN4 bulkhead fitting where it passes through a panel. It threads onto the 7/16 inch-20 bulkhead shank on the far side of the firewall, chassis rail or tank wall and clamps the fitting solidly in place, so the panel carries the load of the line rather than the hose or fitting doing so. Bulkhead nuts are a wear and loss item that often go missing when a fitting is removed, which is why it is worth having spares to hand. Make sure the fitting shank is long enough for your panel thickness, and tighten the nut against the panel without distorting thin sheet.