log4j2 double dollar $$ sign meaning in configuration
<p>I am reading the configuration part of Log4j2.
<a href="http://logging.apache.org/log4j/2.x/manual/configuration.html" rel="noreferrer">http://logging.apache.org/log4j/2.x/manual/configuration.html</a></p>

<pre class="lang-xml prettyprint-override"><code>&lt;Appenders&gt;
    &lt;Console name="STDOUT"&gt;
      &lt;PatternLayout pattern="%m%n"/&gt;
    &lt;/Console&gt;
    &lt;List name="List"&gt;
      &lt;ThresholdFilter level="debug"/&gt;
    &lt;/List&gt;
    &lt;Routing name="Routing"&gt;
      &lt;Routes pattern="$${sd:type}"&gt;
        &lt;Route&gt;
          &lt;RollingFile name="Rolling-${sd:type}" fileName="${filename}"
                       filePattern="target/rolling1/test1-${sd:type}.%i.log.gz"&gt;
            &lt;PatternLayout&gt;
              &lt;pattern&gt;%d %p %c{1.} [%t] %m%n&lt;/pattern&gt;
            &lt;/PatternLayout&gt;
            &lt;SizeBasedTriggeringPolicy size="500" /&gt;
          &lt;/RollingFile&gt;
        &lt;/Route&gt;
        &lt;Route ref="STDOUT" key="Audit"/&gt;
        &lt;Route ref="List" key="Service"/&gt;
      &lt;/Routes&gt;
    &lt;/Routing&gt;
  &lt;/Appenders&gt;
</code></pre>



<p>What is the meaning of double $$ sign? e.g. $${sd:type}?</p>