AppointmentItem.GlobalAppointmentID Property (Outlook)
Returns a String value that represents a unique global identifier for the AppointmentItem object. Read-only.
There are situations where the entry ID of AppointmentItem objects may change, such as when an item is moved to a different folder or to a different store. Entry IDs can also change when a user performs certain functions in Outlook, such as exporting and then reimporting data.
Therefore, each Outlook appointment item is assigned a Global Object ID, a unique global identifier which does not change during those situations. The Global Object ID is a MAPI property that Outlook uses to correlate meeting updates and responses with a particular meeting on the calendar. The Global Object ID is the same across all copies of the item.