The Alerts tab show the Alerts that have been raised in this session.
The filter buttons, shown immediately above the tree, allows you to restrict which alerts are displayed:
Allows to show only the alerts of URLs that are in scope.
Allows to show only the alerts of the Sites tab selected tree node.
Note: Selecting one filter will disable the other as they are mutually exclusive.
Right clicking on a node will bring up a menu which will allow you to:
This will bring up the Resend dialog which allows you to resend the request associated with the alert after making any changes to it that you want to.