The 23 cm height is the length of the sleeve from its top edge to its bottom edge when laid flat. In this model, that height means the sleeve covers a broad zone above and below the knee rather than just a narrow band around the kneecap. Shorter knee sleeves cover less of the thigh and calf, which some people find easier to put on but also easier to slide out of place. With a 23 cm sleeve, the fit is distributed over a larger area, so the pressure feels less concentrated and the sleeve has more surface to hold onto during movement.
This matters when you choose a size and when you put the sleeve on. The top edge should sit comfortably on your lower thigh and the bottom edge on your upper calf, without the edges rolling or cutting in. If your leg shape is much larger above the knee than below, the height can make a well-fitted sleeve harder to pull over the calf; in that case, take your time and ease it up gradually rather than forcing it. The broader coverage is part of why size 5 is described as a roomier fit for people who need extra space while keeping the sleeve in place.