Novelty is a small addon that adds an additional equipment slot to the player. It adds 24 new slots, including a hat, face, neckless, 2 bracelets, 2 hands, 2 feet, belt, back, spellbook, 2 charms, and 10 trinkets. This addon doesn't add any accessories; it is intended to be used by other addons.

compatibility:

if you want to add your items to be compatible with accessories slot, you have two opitions to make your own compatibility by usings tags and commands or scriptings with additional library.

option 1: using custom tags

you can add those tags to your custom items:

"novelty:is_hat": Add item can be eqipped at hat slot, 
"novelty:is_belt": Add item can be eqipped at belt slot, 
"novelty:is_face": Add item can be eqipped at face slot, 
"novelty:is_hand": Add item can be eqipped at hand slot, 
"novelty:is_bracelet": Add item can be eqipped at bracelet slot, 
"novelty:is_trinket": Add item can be eqipped at trinket slot, 
"novelty:is_back": Add item can be eqipped at back slot, 
"novelty:is_neckless": Add item can be eqipped at neckless slot, 
"novelty:is_foot": Add item can be eqipped at foot slot, 
"novelty:is_spellbook": Add item can be eqipped at spellbook slot, 
"novelty:is_charm": Add item can be eqipped at charm slot

player has equipped accessories slot will have tag "novelty:<item identifier>",

example code:

items/test_item.json

{ 
  "format_version": "1.20.80", 
  "minecraft:item": { 
  "description": { 
    "identifier": "example:item_hat", 
    "menu_category": { 
      "category": "equipment"
    } 
  }, 
  "components": { 
    "minecraft:icon": "brush", 
    "minecraft:tags": { 
    "tags": [ 
        "novelty:is_hat" 
      ] 
    }, 
    "minecraft:max_stack_size":  1 
    } 
  } 
}

and the command sample uses a tag selector:

execute at @a[tag=novelty:example:item_hat] run effect @s levitation

option 2: using custom library scripting

using a scripting library can be able to define vanilla items to accessouries slot, by using scripting you can also interacting into the item like see dynamic properties, durability, and also replacing the item.

you can copy the library inside the script behavior pack at path "scripts/lib/NoveltyManager.js".

here is some little documentation:

Interface AccessoriesSlot:
    Hat: "Hat"
    Belt: "Belt"
    Face: "Face"
    Hand: "Hand"
    Bracelet: "Bracelet"
    Trinket: "Trinket"
    Back: "Back"
    Neckless: "Neckless"
    Foot: "Foot"
    Spellbook: "Spellbook"
    Charm: "Charm"

Class NoveltyManager:
    static registerAccessoriesItem(identifier, slot): return void
        identifier: string, eg: "minecraft:apple"
        slot: Interface AccessoriesSlot, eg: AccessoriesSlot.Hat

    static registerAccessoriesItemFromTag(tag, slot): return void
        tag: string, eg: "minecraft:is_sword"
        slot: Interface AccessoriesSlot, eg: AccessoriesSlot.Hat

    static getAccessories(player, slot, index = -1): return ItemStack | [ItemStack] | undefined
        player: Class Player
        slot: Interface AccessoriesSlot, eg: AccessoriesSlot.Hat
        index: int, eg: 1 
(index is suppose to select the index of the item, like hands with index 0 is left and 1 is right, also with trinket slot can select from 0 - 9, based on total amount of the slot. index -1 is mean to all of the items using array)

    static getAccessoriesSlot(player, slot, index = -1): return ContainerSlot | [ContainerSlot] | undefined
        player: Class Player
        slot: Interface AccessoriesSlot, eg: AccessoriesSlot.Hat
        index: int, eg: 1 
(index is suppose to select the index of the item, like hands with index 0 is left and 1 is right, also with trinket slot can select from 0 - 9, based on total amount of the slot. index -1 is mean to all of the items using array)

    static getAllAccessories(player): return [ItemStack]
        player: Class Player

    static getAllAccessoriesSlot(player): return [ContainerSlot]
        player: Class Player

    static setItem(player, slot, itemStack): return void
        player: Class Player
        slot: Interface AccessoriesSlot, eg: AccessoriesSlot.Hat
        itemStack: Class ItemStack

example code:

import { NoveltyManager, AccessoriesSlot } from "./lib/NoveltyManager"; 
import { system, world } from "@minecraft/server";

NoveltyManager.registerAccessoriesItem("minecraft:apple", AccessoriesSlot.Face);

system.runInterval(()=>{
    for(let playerData of world.getPlayers()){
        let face = NoveltyManager.getAccessoriesSlot(playerData, AccessoriesSlot.Face);
        if(face != undefined && face.typeId == "minecraft:apple"){
            playerData.runCommand("say using apple")
        }
    }
}, 5);

if you want to make it simple I recommend using option 1 but if you using more complex I recommend using option 2

Novelty API