OpenXR is composed of the core feature-set, multi-vendor extensions, and vendor-specific extensions. One of those vendors-specific extensions is VARJO_quad_views, available on Varjo HMDs as the name of the extension indicates. By default, in OpenXR, an application will generate two images, one for each eye. This is known as stereo views. But one unique capability of Varjo HMDs (except the Aero), is to have two displays per eye, allowing human eye resolution in the center area of the screen. The purpose of VARJO_quad_views is to enable the application to generate one view covering the entire field of view at the resolution of the context screen, and one inner view to match with the resolution and field of view of the focus screen (padded for taking into account the head movement and latency).
The VARJO_foveated_rendering extension is augmenting the VARJO_quad_views extension by adding eye tracking. The idea is that every rendered frame the application will be provided a different position of the inner view based on where the eyes are looking at. But also, since we know where the eyes are looking, the size of the inner view is much reduced from the default quad_views. And then the resolution of the outer view can also be reduced, as there is no need to even calculate as many pixels as the context screen can display, because even though it is not as high resolution as the focus screen, it is still higher resolution than the eye can detect at its periphery.