I was able to do some research and according to the manufacturer's instructions, "HD" stands for "HD hinge", which allows larger monitors such as a "Samsung Odyssey G9 49" ultra wide" to be positioned more comfortably. If an ultra-wide monitor is not to be used, the HX version without HD joint can also be selected without further ado, provided the requirements (weight, display size, VESA, etc.) are suitable.