Topic category: User side tutorials
First of all, a lot of you may think that setting biome weight to 1024 (the maximum possible) will set it to cover the whole world. Nope. Not correct.
Suprisingly, it actually works with the temperature of a biome to decide how rare it is. This may get quite complicated, but i'll try to explain it as simply as possible.
To begin with, you know how biomes are coded to spawn near other biomes of the same temperature? (like desers + savanna + jungle all spawning close to each other, same goes for forests and icy biomes etc.) So "warm" biomes will spawn near other "warm" biomes. This is actually how biome weight works. Also, note how when you add a mod that adds overworld biomes, all the seeds seem to change; biomes will switch places, etc. However, (i'm not 100% sure, but), we have reason to believe that the temperature zones stay the same. Here is my reasoning behind this:
So I created a biome called "IceFlats" and obviously I set it to "icy" temperature type. I set biome weight to 1024. I loaded up a random seed. I couldn't see it. This is because I spawned in a plains biome, a "warm" biome. Therefore there is a high chance I wouldn't be able to find it nearby. I then loaded up a seed which spawns you next to a huge range of ice biomes (cold taiga, ice plains, ice mountains, ice plains spikes etc) and guess what? All of these biomes were gone, only my "IceFlats" biome was there. That is why I think that the temperature zones stay the same. Likewise, if I were to create a "desert" temperature biome, and set biome weight to 1024, all other biomes with the "desert" temperature will not spawn. So if you set it to 512, around half of the other biomes will spawn, and the other half would be the created biome.Obviously setting it to 0 would make it not spawn whatsoever (useful for dimensions).
Confused? Setting biome weight to the max will cover all biomes the same temperature as the one you created. Setting it to 1 would make a tiny amount (1/1024 I think) of all biomes with the same temperature as the custom one, your custom biome. This gives you an idea on the rarity.
A full biome temperature list is https://minecraft.gamepedia.com/Biome so you can see what biomes you might replace which have the same temperature as your custom biome.
I hope this helped you to understand how biome weight works and what it means! :)