The OnProgressUpdate event (see screenshot below) is available on sub pages only. This is because the event is relevant only for those subpages that report progress of server actions.
The OnProgressUpdate event is triggered when the progress of server actions has moved forward another step as defined by the Progress Update action. This action updates, via its Value property, the \$MT_Progress variable. You can now use the variable in the actions of the OnProgressUpdate event. For example, in the screenshot above, a node in one of the page sources of the Progress subpage has been updated with the value contained in \$MT_Progress. Since this page source node would be updated continually—in fact, each time the Progress Update action is executed—it will contain a continuous record of the server-action progress, and you can display this progress information on the subpage.
See the Progress Indicator tutorial for an example of how the OnProgressUpdate event is used.