title: What is the use of allowBlank: false in extjs fields
body: <p>I have a textfield in a Panel. For example:</p>
  
  <pre><code>{
      xtype:'textfield',
      id:'usap8',
      width:199,
      height:26,
      colspan:1,
      margin: '1 0 1 0',
      allowBlank: false,
      fieldStyle: 'text-align: center;'
  }
  </code></pre>
  
  <p>Will I be able to restrict the panel submission? If not, what is the use of <code>allowBlank</code> config?</p>