Program is open source; code is linked at the bottom of the first post.
https://github.com/kwsch/pkNX/blob/1dad93d7c5e9dd7901930782225ba8c9caa08e6e/pkNX.Structures/Encounter/Nature.cs
Generally, if the game uses an out-of-bounds value that is -1 or 1 greater than the usual range, it implies the game to randomize it when generating in-game.
For other magic numbers that are not translated to not-numbers / "english", check other entries to see a pattern. Usually that's good enough to see what the value does. And if not, try checking for other game's usage of that type of value:
https://github.com/kwsch/pkNX/blob/e432370a407dfd15f8f4dc0463e456ba720cd154/pkNX.Structures.FlatBuffers/Arceus/Util/ShinyType8a.cs#L8