Plug in xml question

I am looking at one of the plugin xml
<service name="InetAddress Ping"
             description="Java InetAddress Monitor">

      <config>
        <option name="hostname"
                description="Hostname"
                default="localhost"/>

        <option name="sotimeout"
                description="Socket Timeout (in seconds)"
                default="10"
                type="int"/>
      </config>
  
      <plugin type="collector"
              class="InetAddressChecker"/>

      <plugin type="log_track"/>

      <metric name="Availability"
              template="${sockaddr.template}:${alias}"
              indicator="true"/>

      <metric name="Response Time"
              template="${sockaddr.template}:${alias}"
              indicator="true"
              category="Throughput"
              units="ms"/>
</service>
Metrics refer to an alias variable. But I do not see any reference to this ${alias} anywhere in the plugin xml. Please help me understand this.
In the metric,
name is "Response Time"
then is alias= "ResponseTime" without spaces.

Thanks in advance.