title: What is the meaning of "version" field in composer.json?
body: <p>In the composer.json it is possible to specify a "version" field; here is an example:</p>
  
  <pre><code>{
      "name": "vendor/dummy_package",
      "description": "Dummy package",
      "version": "1.0.0"
  }
  </code></pre>
  
  <p>Which is the meaning of this field since it doesn't seem to be mandatory?</p>
  
  <p>Is it used to compare required versions of the <code>vendor/dummy_package</code>?</p>
  
  <p>In that case, does it win over tag?</p>
  
  <p>Thank you!</p>