This is an end cap for a 4 inch square PVC post. It's different from normal end caps in that it fits inside the end of the post instead of on top of the end. I'm using it on the bottom of a post that will be sitting on the ground. It was made to fit inside so that it isn't visible. In my case, I'll have water inside the post so I wanted it to be water tight with no leaks. If you print it with 100% infill, and use a liberal amount of PVC cement when attaching it, it shouldn't leak.