Summary:

It is possible to control the flow of Task and Appointment records, between CRM and Outlook, in both directions: in Administration -> Users -> User Configuration, the 'One Way Synchronisation' option can be set to 'No', 'From Outlook to CRM only' or 'From CRM to Outlook only'.

But you can also be more specific than this, by using the 'Comm_CRMOnly' field, in the 'Communication' table.
This defaults to 'NULL', but, if set to '1', prevents any given Communication record from synching to Outlook, at all.

This is useful, because it allows you to use a TableScript, hidden field or equivalent, to update this column when a Communication record is saved, if particular criteria are fulfilled: you may, for instance, not want to synch Communication records with an 'Action' of 'Phone In' to Outlook. 

More Infromation:

If you do set the 'Comm_CRMOnly' flag, it will override the option you selected for 'One Way Synchronisation', in Administration -> Users -> User Configuration.