title: What is the appropriate status code if the user has not provided valid input, as well as an e-mail address that is not available?
body: <p>According to some questions and answers, an appropriate code for cases where a username, for example, is too short, does not meet given requirements, etc., is <strong>422</strong>, even though a lot of recommending <strong>400</strong> as well.</p>
  <p>The status code advised when an e-mail address is in use is <strong>409</strong>, once again according to other posts.</p>
  <p>Which one should I return if the entered username is too long and the e-mail address is not available? Aren't both technically correct? Which one should be the dominant one?</p>