Hi Priyanka,
@logDate(from nms:trackingLogEventHisto) means a DateTime when someone opens the email or clicked some link in the email that you sent from the message center.and event/@created means when that event is created(the time when some made a soap call to trigger this event)). You can have multiple logDate per email but you will have one event/@created per event triggered in adobe campaign via Message center.
Regards,
Amit
