heyas; okay, this is the simple explanation: you know that vue tutorial/sample thingy, the landscape arch? like where you draw the arch on the terrain, and then clip out the low altitudes, and then flip the terrain up on its side, so you have the arch arching over the landscape? right, okay, what happens when you turn the arch or fly the camera around behind it? you get the flat, chopped off back side of the terrain. the symmetrical lattice makes the flat, chopped off back side of the terrain a mirror image of the nifty, un-flat side of the terrain, and thus makes your terrain thingy all 3 dimensional from all sides and all. the only problem (unless the later patches fixed this) is the vue symmetrical terrain isn't solid all the way through, so if you clip a lot of lower altitudes, or have terrains with high sides, the top doesn't match the bottom, and you get two mirrored terrains with a big hole between them.