Single root domain with a structured OU model

This AD diagram example was redesigned from the picture "Single root domain with a structured OU model" from the book "Active Directory for Dummies".
"A domain is the cornerstone that you lay whenever you create trees and forests. Regardless of whether you design a tree or a forest, the starting point is always the root domain. The root domain is the first domain that you create in your AD structure, and it sits at the top of your diagram.
The root domain of your tree, similar to any other domain, is a grouping of
resources built on the following components:
(1) Domain controllers.
(2) Security policies. ...
For many small and medium-sized companies, a single root domain with a
structured OU (organizational unit) model... provides sufficient flexibility for an AD tree. ...
However, larger companies, companies with complex organization charts, and
companies with multiple sites often find that a single domain isn’t suitable." [Steve Clines and Marcia Loughry, Active Directory® For Dummies®, 2nd Edition. 2008]
The Active Directory diagram example "Single root domain with a structured OU model" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Active Directory Diagrams solution from the Computer and Networks area of ConceptDraw Solution Park.