Pack Developers Guide

First, I highly recommend studying the Items Model Definition Wiki to understand what's going on here.

RPT adds a variable system and tools for working with them. Below is a description of how to work with them.

rpt Field

Now in each json file you can specify an rpt field, which contains flags and variables in the format shown in the code block below. They can be read through specific item models. You can read about this here.

{
  "model": {
    ...
  },
  "rpt": {
    "custom_flags": [ // Additional flags
      "foo",
      "bar"
    ],
    "variables": { // Variables
      "strings": { // Strings
        "foo": "bar"
      },
      "numbers": { // Numbers
        "foo": 42
      },
      "flags": { // Flags (true/false)
        "hello_world": true
      },
      "models": { // Models (from the models/ folder)
        "bar": "minecraft:iron_ingot"
      }
    }
  }
}