title: field started with % in struct in Specman
body: <p>I am new to Specman, just don't know what the &quot;%&quot; prefix of a field in struct mean?
  for example, the codes below.</p>
  <pre><code>struct packet_s like any_sequence_item {
    %addr  : uint(bits: 2);
    %data  : list of byte;
    ...
  };
  </code></pre>
  <p>What's the difference if we removed the &quot;%&quot;?</p>
  <p>Thanks.</p>