[This is preliminary documentation and subject to change.]
Server applications generate the EVENT_SYSTEM_ALERT event to notify accessibility aids that a user interface change has occurred that a user might need knowledge of.
Before Active Accessibility, when an application changed an icon's appearance, accessibility aids had to decide whether or not to inform the user, without a clear idea about the information that the application was presenting. Was the information urgent enough that user's work should be interrupted? Was it moderately important information that the user should be informed about without being interrupted? Or, was it low-priority information that the user could choose to accept or ignore based on preference settings? Now, with Active Accessibility, the accessible object conveys urgency by using object state bits that the client can retrieve by calling the IAccessible::get_accState method.
A visual change normally triggers more than one event. For example, an accessible object generates the EVENT_OBJECT_VALUECHANGE event when its value string changes, and can optionally generate an accompanying EVENT_SYSTEM_ALERT event to indicate to the client that some priority level applies to the previous event.
For more information, see Event Constants and Object State Constants.