When is the UNDO_RETENTION parameter value ignored by a transaction?
when there are multiple undo tablespaces available in a database
when the data file of the undo tablespace is autoextensible
when the undo tablespace is of a fixed size and retention guarantee is not enabled
when Flashback Database is enabled
https://docs.oracle.com/cd/B19306_01/server.102/b14231/undo.htm (undo retention)