Required:
Frequency Categories.
Structure Types.
Knowledge of the Json format.

You can both alter the rules of existing categories and add your own custom categories through the use of .rcnc files by placing them in the ACTIVE folder where your structures are, then viewing them with a text editor (there is no GUI view option for editing this feature). To do either of these, start by generating a category file with /#write natural_generation_category <category> ACTIVE. A new file with the category name will generate in the folder that looks something like this:

{
  "generationInfos": [],
  "defaultSpawnChance": 0.003,
  "spawnDistanceMultiplier": 0.0,
  "spawnDistanceMultiplierCap": 1.0,
  "selectableInGUI": true,
  "title": "Adventure",
  "tooltip": [
    "Structures focusing around combat and loot",
    "Random structure each 330 chunks"
  ]
}

The above file is the "Adventure" category. The following is a breakdown of all entries:

• generationInfos: An array that takes up to three entries; spawnChance, biomeMatcher, and dimensionMatcher. Sets overrides for the default spawn chance. This entry can be left blank like above.
• spawnChance: This parameter is the chance to spawn in the specifically matched entry. Takes a decimal percentage. You can also instead provide it as a spawnChances array (e.g. "spawnChances": [0.2, 0.12, 0.3]). This will cause the category to try to spawn in the biome more than once. A spawn chance of 1 and 1 for example would mean that there would always be 2 structures per chunk.
• biomeMatcher: Takes a biome expression. Sets the biomes to which the spawn chance modifier of this rule applies. Used to increase/ decrease spawn chance in certain biomes. Defaults to "all" if unspecified.
• dimensionMatcher: List of dimensions to which this spawn chance modifier applies. Defaults to "all" if unspecified.
• defaultSpawnChance: This parameter is the default chance to spawn per chunk, and is required. Takes a decimal percentage. You can also instead provide it as a defaultSpawnChances array (e.g. "defaultSpawnChances": [0.1, 0.2]).
• spawnDistanceMultiplier: This is used to make structures in this category spawn more/ less often when further away from spawn. Evaluated as a multiplier percentage. E,g -0.5 means 50% chance less per chunk than the previous, or 0.01 means 1% chance more per chunk.
• spawnDistanceMultiplierCap: The cap for how much the spawn chance modifier can be incremented to. Defaults to 1.0.
• selectableInGUI: Toggles whether users can select this generation category in the in game GUI.
• title: The name of this generation category that appears in the in game GUI.
• tooltip: The tooltip text for this entry that appears in the in game GUI. Takes a comma separated array of strings. I recommend filling this out even if only for personal use. You'd be surprised how often you forget the reason for changes you've made.
• structureMinCap: This determines how many structures are needed in the category for it to fulfill its spawn potential. Defaults to 20. For example, if it is set to 20, and only one structure is registered in the category, it will spawn 1/20th as often as it would if 20 structures were registered. This prevents the same few structures spawning too frequently/ spaces identical structures out more if the category doesn't have a lot of structures.

Below is an example for a custom generation category which has a default spawn chance of 0.1%. This chance is reduced to 0.05% (half as often) if the structure is generating in hot biomes, and is increased to 2% if it's generating in the minecraft plains biome, but only in the overworld. It also never generates in cold biomes. This category also forces a a minimum structure cap of 50, causing all structures in the category to generate less often and spaced further apart if there are less than 50 in it. Structures in this category also generate less often as the player moves away from spawn, eventually reaching a generation rate of 0, disabling them.

{
  "defaultSpawnChance": 0.01,
  "generationInfos": [{
     "spawnChance": 0.005,
     "biomeMatcher": "$HOT"
  }, 
  {
     "spawnChance": 0.02,
     "biomeMatcher": "$minecraft:plains",
     "dimensionMatcher": "0"
  {
     "spawnChance": 0.0,
     "biomeMatcher": "$COLD"
  }],
  "structureMinCap": 50,
  "spawnDistanceMultiplier": -0.5,
  "spawnDistanceMultiplierCap": 0,
  "selectableInGUI": true,
  "title": "Ruins",
  "tooltip": ["Ruins of a civilization centered at spawn", "Random structure each 10 chunks, decreasing density as you get further from spawn"]
}