It looks like all important aspects are mentioned and all details are described, but it still looks very good organized, too, and not confusing or unclear. I like it how even the smallest forms and illustrations are labeled precisely.
In my point of view, the general meaning/ function of these signs is to show the reader that one topic/ chapter is over. Furthermore, it has also a visual function; The size of the frame is on every page the same, but if the author didn’t use these designs the undermost part of the page would be empty. So, they kind of complete the pages where there’s not as much text written as on other pages.