Terrain is the ground you walk on, the barrier stopping you from falling into the void. It is a single piece that flows throughout the entire map and combines multiple layers to complete what the terrain looks like, as well as what effects it has on the player.
The "Alpha Layer" is the transparency layer that determines if the cosmetics of the terrain layer are switched On or Off. Meaning the texture on the terrain layer is made transparent to allow for entry/exits through this layer for circumstances such as Caves and Monuments. This does not impact the physical properties of the terrain layer.
Example: Gas Station has an underground ditch inside the monument with an underground cave. Instead of modelling the terrain layer to fit around this ditch you apply the Alpha layer to just erase the terrain within the area that the intrusion covers.
If you do not apply the Alpha layer you will end up falling through the terrain into the ditch and be under the terrain.
Monuments/Prefabs handle the physical collision for the terrain layer. So adding Alpha to an area without one of these prefabs will result in a see-through hole that you can stand on and not fall through.
There is a "Terrain Trigger" volume prefab that will replicate this circumstance and remove the collider for the terrain layer, allowing players to pass through. Ensure you use terrain triggers everywhere underground or players will be killed by anti cheat.
Ground Splat is what you see most of the time while exploring the map. Besides of making the terrain look prettier, it is important to make some objects, food and other stuff spawn on a map properly. Most of the time ground splat affects something only when it's mixed with a certain topology. To know which ones should be used to get something spawning, visit the Topology page.
Some splat textures has their own details (clutter) that may change. This depends on topologies as well.
Biomes handle a variety of attributes including:
- Character Temperature
- Tree type/model
- Foliage type/model
- Ore Spawning
- Collectable Resource Spawning
- Collectable Food Spawning
Although some changes are purely cosmetic, biomes have influence on the gameplay of your map. Having your whole map "Arctic" biome will result in no food/hemp spawns on your map and no place for a valid spawn area. Keep this in mind when working with your Biome layer. You should try and include all types of Biomes in your map.
Topology is a very important part of every map. They define where trees, bushes, junk piles and other content will spawn on your map. There is a large list of all topologies, what they do and how to get some basic stuff working – it will be much easier to cover everything on its own page rather than making this article awfully long and complex, so visit this article for more details: Topology
Information gathered provided by the Rust Map Making community.