WeChat.ServerMessage.XmlMessage.transfer_customer_service

You're seeing just the function transfer_customer_service, go back to WeChat.ServerMessage.XmlMessage module for more information.
Link to this function

transfer_customer_service(to_openid, from_wx_no, timestamp)

消息转发客服消息 - 官方文档

<xml>
  <ToUserName><![CDATA[touser]]></ToUserName>
  <FromUserName><![CDATA[fromuser]]></FromUserName>
  <CreateTime>1399197672</CreateTime>
  <MsgType>transfer_customer_service</MsgType>
</xml>
Link to this function

transfer_customer_service(to_openid, from_wx_no, timestamp, kf_account)

消息转发到指定客服 - 官方文档

<xml>
  <ToUserName><![CDATA[touser]]></ToUserName>
  <FromUserName><![CDATA[fromuser]]></FromUserName>
  <CreateTime>1399197672</CreateTime>
  <MsgType>transfer_customer_service</MsgType>
  <TransInfo>
    <KfAccount><![CDATA[test1@test]]></KfAccount>
  </TransInfo>
</xml>