<!-- Uncomment this property if you need the extra appender -->
  <!-- property name="EXTRA" value="ON" /> -->  
  <if condition='isDefined("EXTRA")'>
    <then>
      <appender name="EXTRA_APPENDER" class="ch.qos.logback.core.rolling.RollingFileAppender">
      ...
      </appender>
    </then>
  </if>
  ...
  
  <root level="INFO">
    <appender-ref ref="CONSOLE" />
    <appender-ref ref="FILE" />
    <if condition='isDefined("EXTRA")'>
      <then>
        <appender-ref ref="EXTRA_APPENDER" />
      </then>
    </if>
  </root>