title: What is the difference between DBInstanceIdentifier and DBName for rds.create_db_instance?
body: <p>What is the difference between the parameters 'DBInstanceIdentifier' and 'DBName'?</p>
  
  <p>For a mysql db, it seems to me they are essentially identical.</p>
  
  <p>And since 'DBName' is optional, I can only just pass 'DBInstanceIdentifier' to <code>rds.create_db_instance</code>. Is it right?</p>
  
  <p>In what situation do I need to provide both parameters? And in what situation their values have to be different from each other?</p>