If you delete an appointment in iinsight that was created in Outlook - it will resync this appointment later (as it still exists in Outlook where it was created).
If you delete an appointment in Outlook that was created in Outlook - Deletes from both system
If you delete an appointment in iinsight that was created in iinsight - it will be deleted in Outlook.
If you delete an appointment in Outlook that was created in iinsight - it will be cancelled in iinsight.