what is the metadata attribute pair in MQFTE ? Why is it used?

I am working with wmqfte. While creating a transfer there is a parameter for metadata attribute pair. Why is this used for ?

One of the FTE engagements I worked on required email notifications of transfer status. Our approach to this was to add an email step in the transfer and the way we passed in the source and destination addresses was with metadata pairs. The transfer XML is provided below for an example:

<?xml version="1.0" encoding="UTF-8"?><request version="4.00" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="FileTransfer.xsd">
<!DOCTYPE request>
  <managedTransfer>
    <originator>
      <hostName>host.example.com</hostName>
      <userID>me</userID>
    </originator>
    <sourceAgent QMgr="QMGR01" agent="AGENT01"/>
    <destinationAgent QMgr="QMGR02" agent="AGENT02"/>
    <transferSet priority="5">
      <metaDataSet>
        <metaData key="email.from">fteadmin@example.com</metaData>
        <metaData key="email.to">"dept@example.com</metaData>
      </metaDataSet>
      <item checksumMethod="MD5" mode="binary">
        <source disposition="delete" recursive="false">
          <file>/root/path/file</file>
        </source>
        <destination exist="overwrite" type="directory">
          <file>/root/path/</file>
        </destination>
      </item>
    </transferSet>
    <job>
      <name>Your Job Name Here</name>
    </job>
  </managedTransfer>
</request>

A better way of sending status emails is to watch the transfer notifications published at the Coordination QMgr. However this example does show one possible use for the metadata pairs.