its more that they divide the texture into a grid of all textures needed for a render call. (according to this thread @ GameDev.net http://www.gamedev.net/community/for...opic_id=460053)
They have some textures where they put up the mipmaps and textures used for that render call. As carmack said: we can render the entire scene in 3 render call (don't ask me how) if we disable culling n stuff...