A map needs two informations: the first one is the .sco file, which contains all the modifications that you made on your map (terrain height, texture, props). The second one is the terrain code, which is contained in the scenes.txt file. The terrain code determines the default shape of your map.
A good mapper will always choose a custom terrain code, because all modifications to the terrain will increase the weight of the map file, and you know that players hate loading big maps. For example, if you want to make a map that is rather flat, you won't use mountainous terrain! Otherwise you would have to flatten it, and that would sharply increase the weight of your map. If you want to make a small map, you won't use a big terrain for the same reasons. A map with an untouched terrain and with nothing added to it will always weight 0kb.
Installing the terrain code is simple: set it in your scenes.txt file, then go on your map and "reset" the terrain.
So let's have a look at the
Terrain Code Generator
At the top of the page, you will find the terrain code itself. Example: 0x00000001324a9cba0005194a000041ef00005ae800003c55
Place river: If you want to have a river on your map instead of making one yourself.
Deep water, shade occlude, disable grass: Don't use that.
Polygon size: A very important data. Your terrain is made of different polygons. The bigger the polygons, the harder it will be to polish the terrain with fine details. However, big polygons will most likely increase the performance on your map (too many polygons will hurt your hardware). Moreover, if you choose 2 polygons, you won't be able to generate AI meshes (the minimum for AI meshes is 3). So if you want to make a big map with few terrain details, you will use 4 polygons.
Terrain: this is the type of terrain that will be used defaultly on your map. There again, it's especially important to choose the right terrain type. Indeed, if you want to make a snowy map, that would be a very bad idea to paint your plain map with snow texture for 2 reasons: the first one is that your plain has some green overlays over it. So if you paint it with snow texture, your snow will be greenish in some places. That won't happen if your default terrain is snow. The second reason is that texture painting would sharply increase the weight of your map. Also note that the terrain will affect the color of le lightning on your map. A white wall will be rendered greenish with a plain terrain and yellowish with a desert terrain.
Vegetation: If you set a number over 0, it will generate some vegetation. If you want to have a completely custom map, don't use this.
Ruggedness: If you want to have a map with some rock textures, use it. But I advise to avoid using it. Indeed, you will notice that rocky textures are bugged as they don't have smooth edges. It's probably better to draw it yourself.
Valley: If you want to have a map with few hills, set a high number to this.
Hill height: If you want a flat map, you must also set a low hill height. So if you want to have a completely flat map, set Hill Height to 0.
Size y, x: this is the length and width of your map. It's not possible to make a map bigger, you would obtain a black terrain. You can decrease the size of a map, but it's better to avoid doing that.