If you come across this document, it's written by a dev.

The reason why there's so many lines in spawn.json is for Skeleton and Zombie armor, they're a bit different.
There's 3 stages in this modpack:
1. pre-nether (when it's not mentioned, that's the default one)
2. post-nether
3. post-end

Zombies and Skeletons have custom armor on each of those stages. That's 6 configurations, since Zombies and Skeletons have different light touches to them (one is ranged, one is melee).
These 6 configurations take about 1500 lines. No, these were not written by hand.

An excel spreadsheet contains some information about weights and bases everything on the fact that "empty : true" is a weight of one, which it is for some reason. If you do the math, you get those weight numbers.

Dump that into csv, parse with any language (for this pack, C#), and format into JSON raw text. Then it's a bunch of copy-paste or adjusting into the InControl spawn.json.

But monsters can normally spawn with (sophisticated) backpacks, which InControl will overwrite even if "empty : true". So in that case it's simple: double the rules, and use each first rule as the "filter".

For example if Zombies have 20% chance of armor, then rule one will work 80% of the time with the word "random : 0.8". That first rule doesn't have a chestplate, and therefore allows backpacks.
If it's the second rule that's chosen, the 20% where they wear armor, then we remove the "empty : true" so a chestplate is guaranteed.

Effectively, it's a bit redundant for helmet/leggings/boots, but it means that they really have 20% chance of custom armor for that stage, and when they don't have a chestplate, anything including a backpack can replace it.

It's still possible for the backpacks to try and spawn on the mobs regulated by InControl (those that still have a chestplate), so that's why HP buffs are disabled in sophisticated backpacks (else random mob looks like it has buffs for no reason). I've adjusted the spawn rate of backpack mobs a tiny bit to compensate, and drop rate cause fun.

-R3GEN