You can replace the infantry with mountaineers or marines if you want to have special forces. You should make this if your using the superior firepower doctrine as the allies. Like as Italy and the UK you can put a couple of these in Africa or as the Soviets and put groups behind the lines to move in and counter pushes. This is AT infantry that you put where you expect tanks to be. You should build this is your not expecting to win the air war just to make the cas less effective against your divisions. This is the same as the basic infantry except it has AA. I use them on Germany and Italy to fill lines and defend. You can use these as coastal defense or just fillers in a line if you have tanks.