Examine the parameters for a database instance:
Your database has three undo tablespaces and the default undo tablespace is not autoextensible.
Resumable space allocation is not enabled for any sessions in the database instance.
What is the effect on new transactions when all undo space in the default undo tablespace is in use by active
Transactions write their undo in the SYSTEM undo segment.
Transactions wait until space becomes available in UNDOTBS1.
Transactions write their undo in a temporary tablespace.
https://docs.oracle.com/cd/B19306_01/server.102/b14231/undo.htm (undo retention)