lib/paper_trail/model_config.rb
def check_presence_of_item_subtype_column(options)
      return unless options.key?(:limit)
      return if version_class.item_subtype_column_present?
      raise format(E_MODEL_LIMIT_REQUIRES_ITEM_SUBTYPE, @model_class.name)
    end
def check_presence_of_item_subtype_column(options)
      return unless options.key?(:limit)
      return if version_class.item_subtype_column_present?
      raise format(E_MODEL_LIMIT_REQUIRES_ITEM_SUBTYPE, @model_class.name)
    end
Some options require the presence of the `item_subtype` column. Currently
 only `limit`, but in the future there may be others.

 @api private