In the Classes section, you declare an Order class as follows:

class Order < ActiveRecord::Base

But at the end of the paragraph, you state "This Order class is defined to be a subclass of the ApplicationRecord class. I’m presuming you mean “… ActiveRecord class”.

I see later (tipped off to look further by the code at the bottom of page 83), while working through the Depot project that we have:
/app/models/application_record.vb

class ApplicationRecord < ActiveRecord::Base
  primary_abstract_class
end

and then:
/app/models/product.rb

class Product < ApplicationRecord
end

So I’m guessing that this eventual set up is where that initial ApplicationRecord came from.