For instances with many projects it is not only required to have a hierarchy of projects you should also have a reporting functionality on project hierarchy. Sub-projects dashboard item allows you to visualize issue distribution on project hierarchy level. You can customize which part of project hierarchy you want to display or and which JQL results you want to see. Clicking on any issue count opens "Issue Navigator" to show actual issues. Using this gadget you can create high level combined project status reports for related projects.
