They are actually the nuts that a bolt screw into that leave a flush rounded head usually slotted for a Philips head screwdriver. When ever I want to mount something to the deck that won't be taking tons of force and don't want to through bolt it I use these. But not the way they were designed to be used. I use a hole saw with a pilot drill cut off almost flush to cut through the outer deck and wood core. Then I take a three inch #8 wood screw and cut off the head and bend it in a vice 90 degrees with the bend at about 3/8 of an inch from the tip. I put the bent screw into a 1/4 inch drill and put the tip into the hole and use the screw to route out the wood back under the deck the 3/8 inch. Then I take the barrel bolt and set it into the hole using a large fender washer, nut and regular bolt to set the height so the barrel bolt will be flush with the deck. Then I back fill the hole with epoxy. The epoxy flows back under the deck and makes a plug capturing the Barrel bolt with only the threaded tube left at deck level. You can screw your "whatever" down and never drill through your deck. I've mounted things like small turning blocks for guiding my boom vang line, my SS cage over my heater vent, my deck box and a few other relatively lightly loaded items. I have no idea how much force it would take to tear a 1/4 inch barrel bolt head, which is about 3/4 of an inch across, out of a deck pulling the epoxy plug up through the deck with it but it would be a lot. I use the same technique without the bolt to seal the deck core when through bolting and just leave out the barrel bolt. Once the epoxy sets up I just drill through the plug. That way there is always a solid plug between the mounting bolt and the deck core.