Assembly: System.Web.Extensions (in System.Web.Extensions.dll)
The default value is an empty string, which means that the UpdateProgress control is not associated with a specific UpdatePanel control. Therefore, asynchronous postbacks that originate from inside any UpdatePanel control or postbacks from controls that are triggers for the panel cause the UpdateProgress control to display its ProgressTemplate content.
You can set theproperty to controls in the same naming container, in a parent naming container, or in the page.
Available since 3.5