Used as a standpipe for antenna attachment:
The 45mm standpipe allows mounting of an antenna without attaching the mast to boom assembly directly to the mast. Attach boom-to-mast assembly of desired antenna to the standpipe, and then drop the standpipe over the top segment of the aluminum mast. If the standpipe is slid down mostly or the entire way over the top segment provides additional mechanical strength at the top of the mast also. Makes it easier to attach and antenna that has a hole for insertion of a mast like the Spiderbeam yagi or some hex-beam style antennas.
Hold this tube vertically with the clamp section pointing downward. Mount the antenna to this tube. Slide the tube over the top segment of your aluminum mast and tighten the clamp. Voila! - You are now ready to push up the mast.
With this method, you can keep the antenna and mast two separate items until the very last moment.
This often makes assembly easier but it is not absolutely necessary. Of course you can simply mount your antenna directly to the top segment of the aluminum mast.
This tube segment is suitable for all aluminum masts with a top segment of 40mm diameter:
33ft / 10m HD aluminum mast
47ft / 14.5 HD aluminum mast