This is concerning the Spawn of Psyduck sidemod.
Right now, as I understand, the only way to associate a pixelmon with a certain biome to spawn in is to list that specific biome under their entry in the config file. This works perfectly well if you just want to add a biome for a certain pixelmon to spawn in. However, as soon as the change is for more than a single pixelmon, things start to get really tedious: For example, say, you have a new biome that is an ocean variation and you want all pixelmon spawning in the biome 0 (vanilla ocean) to also spawn in your new biome. That is a monumental task as a simple search and replace will not really work well. And because there's probably about a hundred pixelmon whose entries would need to be adjusted.
So as an addition to the current system, offer a second option: The option to set a biome X as "equal" to another biome Y. Hence, all pixelmon that already spawn in biome Y due to it having been entered into their normal lists, will now also spawn in biome X.
Honestly it's not really an end-all solution to the problem though as it only works with biomes that are similar to an already existing one. But it's the from what I could tell "most simple" method I could think of to solve that particular issue in a satisfactory way.
The way I could imagine it working specifically is in a json file, a list like so:
"associations": {
// ocean
"0": [
201,
202
],
// another biome
"1": [
240,
241
]
}