ash/priv/documentation/topics/attributes.md

  # taken from `attribute.always_select?`
  Useful if fields are used in read action preparations consistently.

  A primary key attribute *cannot be deselected*, so this option will have no effect.

  Generally, you should favor selecting the field that you need while running your preparation. For example:

  ```elixir
  defmodule MyApp.QueryPreparation.Thing do
    use Ash.Resource.Preparation

    def prepare(query, _, _) do
      query
      # we use `ensure_selected` here because we don't want
      # to limit the fields being selected to only this attribute, rather
      # rather we want to make sure this field is definitely selected.
      |> Ash.Query.ensure_selected(:attribute_i_need)
      |> Ash.Query.after_action(fn query, results ->
        {:ok, Enum.map(results, fn result ->
          do_something_with_attribute_i_need(result)
        end)}
      end)
    end
  end
  ```

  This will prevent unnecessary fields from being selected.

Taken from attribute.generated?

If it is, the data layer will know to read the value back after writing.