Diagnose alerts currently have two statuses that enable you to discern between new alerts and alerts that have already been read.
Note: The status for any alert is defined on a per user-account basis. That is, an alert will remain new for your user-account until you review the ticket regardless of if a team member in your organization has reviewed the ticket.
New alerts simply mean alerts that have not been read/opened by your user account. New alerts can be delivered to your email address, viewed from the bell icon, and from the New tab of the alerts page.
New alerts will remain on the New tab of the alerts page from latest to oldest until they are reviewed (marked as read) by your user account at which point they will be moved to the Reviewed tab.
There is also a limit of 80 New alerts that can be accessed from the alerts page at any given time. As alerts are reviewed and moved to the Reviewed tab, the oldest non-visible new alert will be added to the page.
Reviewed alerts are alerts that have been read/opened by your user account. Reviewed alerts can be viewed from the bell icon or from the Reviewed tab of the alerts page.
Reviewed alerts will remain on the Reviewed tab of the alerts page indefinitely.