DELETE ROWS means that the Oracle will truncate the table (all rows are deleted) after each commit.
PRESERVE ROWS means that Oracle will not truncate the table until the end of the session.
Managing Temporary Tablespace for Temporary LOBs
Temporary tablespace is used to store temporary LOB data. As a database administrator you control data storage resources for temporary LOB data by controlling user access to temporary tablespaces and by the creation of different temporary tablespaces.