The Velocity templates for batch notification does not include issue objects which can be used, and we cannot add supposedly correct issue objects in the templates.

For example, IssueUpdateBatcher-header.vm only has issueKey and issueUrl and does not contain the complete issue details such as issue types. So customizing this template is limited to what the template has prepared.

A use case scenario would be to display the parent task in the header of the mail (project / issue / subtask instead of project / subtask).