This is a nice table describing what happens is you combine the AutoSave, AutoUpdate, AutoReplace properties of a dataport, when importing records into a table directly.

If AutoSave is No, it is possible to insert or modify records by using INSERT/MODIFY in the code.   
X means that in this case the setting for this property does not affect the outcome.   
AutoUpdate works by finding the corresponding record in the database and changing or updating only those fields that are defined in the Dataport.  The other fields remain unchanged.