At first, I was thrilled we could make a custom biome list for our new dimensions, but then I ran into problems. Most of the time, the dimension only used one biome! If I was able to somehow force the game to use more, then they would be used incorrectly. However, I finally figured out how these work.

The ability to select biomes comes from the Nether's new multi_noise biome system. It places multiple biomes based on the following variables.

- Temperature
- Humidity
- Altitude
- Weirdness
- Offset

How do you find the correct values? You don't. That's the big secret. There is no right value for a biome. There is only the values that work for you. As far as I can tell, the labels on these values are just that, labels. They are there because us humans prefer things like "Temperture" instead of "Value 0".

How the Values Work:
- NEVER give the same set of values to more than one biome! This is what causes the game to leave out biomes.
- ALWAYS use a number in the range -1 to 1. Anything outside that range may cause the biome to not appear in the game.
- For the main biome, use all zeros. A biome with this setting will be extreamly common.
- For more common biomes, use settings similar to the main biome.
- For more rare biomes, use settings on the more extreme end of the range.
- If you want similar biomes grouped together, then give them similar values.
- If you want a certain biome to be used as a transition between two other biomes, then give that biome a set of values between the two biomes.

Example:
The above screenshot is from my Frozen Wastes datapack. It has three biomes, Ice Spikes, Frozen Oceans, and Deep Frozen Oceans. Everything is 0 in all biomes except the Altitude. Those values are

- Ice Spikes: 0
- Frozen Ocean: -0.25
- Deep Frozen Ocean: -0.5

This causes Ice Spikes to be the main biome and Deep Frozen Ocean to be kinda rare. This also causes Frozen Ocean to act as a transition between Ice Spikes and Deep Frozen Ocean.

Furthermore, I could change Temperature instead of Altitude and use positive instead of negative values and get the same results. This is because the game only cares about how similar the values are among the biomes.