Asked April 23, 2021, 9:51 AM EDT
Hamilton County Ohio
I can see your confusion. Remember Black Raspberries produce fruit on second year canes called floricanes. First year canes are called primocanes. What the fact sheet is telling you to do is to tip prune your primocanes (first year's canes) when they reach 24". Your second year, fruit bearing canes continue to grow and are supported by the trellis at 40 inches.
In case you are interested here is a link to a publication on brambles from Cornell University Extension. You will find it cover all aspects of growing all kinds of brambles. It has a great section on support structures for brambles and black raspberries support structures are covered in detail: http://www.hort.cornell.edu/fruit/berry-guides/bramble.pdf
I wish you the best of luck with your Black Raspberries and what you are growing sounds exciting. Also, hopefully, we are done with the really bad, cold nights and can finally get around to getting our garden growing!