title: UML use case with Login and Registration
body: <p>In my UML Use Case diagram I've got 1 actor(User) and 2 use-cases(Login and Registration). They are connected as in image below(Association between User and Login, and Extend between Login and Registration)</p>
  
  <p><a href="https://i.stack.imgur.com/qB6Xl.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/qB6Xl.jpg" alt="My use case diagram in question"></a></p>
  
  <p>I've read on online sources about UML about the relationships in the use-case diagrams, but I have problem in defining Registration use-case. In which situations can Registration use-case be optional and mandatory?</p>
  
  <p>P.S. Feel free to advise modifications to this diagram.</p>