On older boards way-back-when which only supported 2GB RAM in total this sort of thing referred to what you would expect to see after on-board video hardware and other devices and consumed some.
On a modern board like this (which supports 64GB as the other specs state) this refers to what memory can be allocated to the on-board GPU - so you might put 8GB in there but only see ~6.25 GB available to OS/apps/games as the GPU has claimed the remainder for its work. You usually have some control over what the GPU does claim, and of course if you completely replace its function with an add-on card (which you likely will if building a machine for gaming) it should take 0.
This information is normally listed with or directly after the name/model of GPU in a specs sheet, I assume that line has been missed out by mistake in this case (either a mistake on the seller's part or due to them copying a bad list from the manufacturer's blurb) leading to a little confusion.
Intel Graphics 4000 HD
capability.Intel Graphics 4000 HD
the simple answer is that I am familiar with how theIntel Graphics 4000 HD
works. If you disable it then theIntel Graphics 4000 HD
wouldn't be assigned any memory because its disabled.