title: UML relation between usecases (extend/include)
body: <p>I do not understand well following topic since it is a bit ambiguous from what I read:</p>
  
  <ul>
  <li><strong>Inlcude</strong> is like a reference to next part, the usecase is not completed without it. This part should be referenced from more places otherwise its use has no sense.</li>
  </ul>
  
  <p>But I have seen an example when there is "include" only in some IF statement is true, like:
  <em>Add new product--->include--->Add new manufacturer</em>. //Adds only when doesnt exist. Why there isnt "extend"? Is it because somewhere else could be "Add new manufacturer" used once again?
  Thanks</p>