Retention Guarantee/NoGuarantee

SQL> select retention ,tablespace_name
2 from dba_tablespaces;

RETENTION TABLESPACE_NAME
----------- ------------------------------
NOT APPLY SYSTEM
NOGUARANTEE UNDOTBS1
NOT APPLY SYSAUX
NOT APPLY TEMP
NOGUARANTEE UNDOTBS2
NOT APPLY TEST
NOT APPLY MAKE

7 rows selected.

SQL> alter tablespace UNDOTBS2 retention guarantee;

Tablespace altered.

When we set any undo tablespace in GUARANTEE then oracle not overwrite any UNEXPIRED data.