title: Understanding incremental development model diagram
body: <p>I was reading <code>Software Engineering 9th Edition</code> by <code>Ion Summerville</code>. On the topic of <strong>iteration in process models</strong> I found this process diagram for <strong>incremental development</strong>. I am unable to understand this diagram. 
  According to me,
  From description the next step should be specification then development and then validation and then the final product should be created. Please help me out in understanding this diagram. Thanks
  <img src="https://i.stack.imgur.com/BqktJ.jpg" alt="enter image description here"></p>