Some types of notifications become obsolete over time: incoming messages become read, slots that need people become filled, pending applications get approved. After VSys generates a notification, if there's a five minute delay or more between the notification's generation and its delivery, VSys will re-verify that the notification is not obsolete.
When you've got a list of notifications on screen, you can click on the Re-check applicability link to force VSys to check the notifications for obsolescence; obsolete ones will be automatically marked as No longer applicable.