A type of field of your app's own: what its values are, how they are checked, how they compare and how they read.

defmodule MyApp.Cardbox.Rating do
  @behaviour Cardbox.FieldType

  @impl true
  def name, do: "rating"

  @impl true
  def kind(_options), do: :number

  @impl true
  def cast(value, _field, _opts) when value in 1..5, do: {:ok, value}
  def cast(value, _field, _opts), do: {:error, "is from 1 to 5, got: #{inspect(value)}"}
end

and in your config, beside Cardbox's own types:

config :cardbox, types: [MyApp.Cardbox.Rating]

The types are your app's config, not your Cardbox module's: a value is read in many places that know only its field (a filter, a table's column, a board's card, the JSON Schema an agent reads), as Kotoba's nodes are. A type's name is never one of Cardbox's (Cardbox.Type).

What a type says

Only name/0 and cast/3 are required; the others have defaults.

• kind/1: how values compare, for filters and sorts (Cardbox.Type.kind/1): :text by default. A :list does not sort or group.
• check_options/3: the field's options, checked when the field is added or changed, with the collection, so that an option may name another field. By default a type takes no options.
• cast/3: a value given, checked, as it is kept (JSON): {:ok, nil} for an empty one.
• load/2: a value kept, read for Elixir (Cardbox.get/3). As it is by default.
• plain/2: a value as plain words or numbers: what a table sorts and shows, what Cardbox.Record.to_map/2 gives, and what a field that changes type converts. As it is by default.
• text/2: a value in words for a person, in a table's cell and on a board's card. plain/2's, as text, by default.
• operators/1: the filter operators, those of its kind by default.
• json_schema/1: the JSON Schema of a value, for agents. A string by default.
• words/2: the words of a value in the record's search text. None by default.