If an event subtype is highlighted, the event has other events associated with it. Clicking a highlighted event subtype brings you to an Install Event Details report, which shows all of the sub-events associated with the event you clicked (per computer).
The Install Event Details report is useful primarily to show the connections between a root event and the events it generates.
Approved installers generate locally approved files, and approved files do not generate sub-events on the Install Event Details page. Unapproved installers generate unapproved files (unless previously approved by some other means), and unapproved files do generate sub-events. Newly installed files that are blocked generate Install Event Details.