A 'virtual' texture is a subset of an another texture; defined by rectangular UV extents that refer to an area of source texels. It is used by the RoadMap classes to pack a number of textures into a single "roadway" texture, but could certainly by extended into a general-purpose class in the future.

One thing to watch out for is that a virtual texture won't necessarily tile (repeat) as expected, since it make have other virtual textures adjacent to it in the source texture.