I was looking for the behavior of the Refresh on the underlying data, wondering if the there was a truncate parameter. According to the docs the atomic refresh has to do with how the transaction is handled and not how the data is removed from the table.
from the docs posted above it does not specifiy delete or truncate just
atomic_refresh
If this parameter is set to true, then the list of materialized
views is refreshed in a single transaction. All of the refreshed materialized
views are updated to a single point in time. If the refresh fails for any of the
materialized views, none of the materialized views are updated.
If this parameter is set to false, then each of the materialized
views is refreshed in a separate transaction.
Then after some more searching askTom does state that a
in 10g and above you would use atomic_refresh=>FALSE
that permits us to user truncate+insert/*+APPEND*/
Ask Tom "Complete MV Refresh Uses Delete vs Trunc..."