A "fitting" is the fence industry term for all of the materials required in assembling a chain link fence.
Brace bands work by sliding the band over the terminal post, lowered into the desired position, and then secured with a carriage bolt that is then tightening using a ratchet and hex nut.
In the fence industry, galvanized posts are commonly referred to by their outside diameter (O.D.) measurement. This can be found by measure across the top of your post from one outer edge to the other outer edge. Common sizes used in fencing are as follows: 1 3/8", 1 5/8", 1 7/8", 2 3/8", 2 7/8", 3 1/2", 4", 4 1/2", 5 9/16", 6 5/8", and 8 5/8".
Note, these sizes differ from other industries such as plumbing. The reason for this difference is that the galvanized posts used in the fence industry are actually made in full-length pieces called "tube" which is historically measured by O.D., whereas the types of posts used in plumbing for example are referred to as "pipe" which is commonly measured by internal diameter or nominal diameter.
No, this method of installation is not normally done in the fencing industry. Fencing dome tops are designed with a metal "skirt" that covers the outer top of the post to help prevent water accumulation and other possible issues. If your installation calls for an internal cap, we would recommend looking on Google or other search engines for "pipe plugs" as this is not a standard fence fitting that we carry.