https://support.ptc.com/help/windchill/r13.0.0.0/en/?_gl=1*1xirda6*_ga*MjA1NjM1MzcyNS4xNjc4OTUwMDM0*_ga_1QBT6P6HR1*MTY5MTQxMTAwMy4zMzAuMS4xNjkxNDEzMDM1LjAuMC4w*_ga_CBN5QVB9VJ*MTY5MTQxMTYwNi4yMTEuMS4xNjkxNDEzMDM1LjAuMC4w#page/Windchill_Help_Center/customization/WCCG_BusLogicCust_WorkflowAdmin_EmailNotif.html
Documentation is above. You need to customize the codebase/templates/General*.html files like below. This is mine to show the PBO name and number. The pboNumber is a workflow variable so you can call those out too. This modifies the email subject as it appears.
<html>
<!-- bcwti
--
-- Copyright (c) 1997, 1998 Windchill Technology, Inc. All Rights Reserved.
--
-- This software is the confidential and proprietary information of
-- Windchill Technology. You shall not disclose such confidential information
-- and shall use it only in accordance with the terms of the license
-- agreement you entered into with Windchill Technology.
--
-- ecwti
-->
<SCRIPT LANGUAGE=Windchill> <!-- beginSubject --> </SCRIPT>
<SCRIPT LANGUAGE=Windchill> <!-- getActivityName --> </SCRIPT>
<SCRIPT LANGUAGE=Windchill> <!-- addText text=" - " --> </SCRIPT>
<SCRIPT LANGUAGE=Windchill> <!-- getProcessVariable varName="pboNumber" --> </SCRIPT>
<SCRIPT LANGUAGE=Windchill> <!-- addText text=" - " --> </SCRIPT>
<SCRIPT LANGUAGE=Windchill> <!-- getPrimaryBusinessObjectName --> </SCRIPT>
<SCRIPT LANGUAGE=Windchill> <!-- endSubject --> </SCRIPT>
<head>
<SCRIPT LANGUAGE=Windchill>
<!--
getStyleSheetProps
-->
</SCRIPT>
</head>
<body>
<font face="Arial, Helvetica, sans-serif">
<SCRIPT LANGUAGE=Windchill> activityNotificationUrl </SCRIPT>
</font>
<SCRIPT LANGUAGE=Windchill>
<!--
activityAttributes notification=true
-->
</SCRIPT>
</body>
</html>
Next I modified the task Instructions to include data which would appear in the subject of the email:

The final result is like this:
