Select ItemModel

A Select ItemModel, is a set of conditions based on a specific property on the Item, that decided when to show what Model.
Vanilla makes use of this in Crossbows to show the firework & arrow when the bow is charged.

There are other ways to use this as it has several properties it can use for this condition.
Recommend to check the Minecraft Wiki for an always up-to-date list.

Below is an example for showing a different Model if an item has been renamed by a player;

custom_diamond:
  ItemModel:
    type: select
    property: component
    component: custom_name
    cases:
      - when: "Renamed Diamond"
        model:
          type: model
          #References assest/namespace/models/renamed_diamond.json
          model: namespace:renamed_diamond
      - when:  # When can also be a list of conditions here
          - "Diamond1"
          - "Diamond2"
        model:
          type: model
          model: namespace:multiple_diamonds
    # A Fallback ItemModel in case none of the cases match
    fallback:
      type: model
      model: namespace:custom_diamond

Info

You can also chain these to use different ItemModel-types inside each case here for very complex items