The basic graphics SW support for a graphics chip (GPU) includes one xscreen per GPU. Until Marox introduced the "DualHead" card with the G400, graphics cards supported one monitor channel per card. With the DualHead card, there were two monitor channels driven off the single GPU.
What was displayed on the two monitors was often the same xscreen, i.e., both monitors showed the same view. Now we call that the "copy mode."
In order to show someting on one monitor that was independent of the view on the other, the capability of having two independent xscreens per graphics chip was developed.
When the multiple-xscreen-per-GPU is present in a Summit Series/Edition, both xscreens are fully hardware accelerated, meaning that the SW uses available hardware on the graphics chip/card to speed up operations.
Newer graphics cards can have GPUs that "drive" four monitors. This allows considerable flexibility in setting up am "X Display". (See "MultiChipSLS" for the X definition of a "Display"). There are a few models of graphics card that are built with two GPUs, each of which can support four monitors/projectors, allowing a single card to "drive" eight outputs. Accelerated-X Summit HX Series allows one to configure the outputs in a wide variety of displays, while still providing full hardware acceleration and high-system performance.
Wnen the Multiple Xscreens per GPU feature is available, it is used as a base for implementing other featrues, such as MultiView and Multi-GPU SLS.