for those taht didnt know this is a "" from the h2vek help file.

Technology Differences between Halo 2 Map Editor and the HEK
Photon Mapping: New lighting results in more robust, accurate, and visually stunning maps. Use a less-intensive lighting setting when testing map changes.
Material System: The improved material system has more options and flexibility. Shader file extensions no longer determine shader type. Shader templates define shader material and are referenced by the unified .shader file. Users now have more options for Shader templates and improved data fields for shader files in Halo 2 Guerilla.
Instanced and Detail Geometry: Instanced and detail geometry allows the creation and placement of geometry that does not have to be stitched into the world.