Server Administration :: System Tablespace Had Been Used More?

In my database ,the size of the system tablespace is 1024M.now i found its free space is 477.875M.

SQL> SELECT TABLESPACE_NAME,SUM(BYTES)/1024/1024 M FROM DBA_FREE_SPACE where tablespace_name = 'SYSTEM' GROUP BY TABLESPACE_NAME ;
TABLESPACE_NAME M
--------------- -------
SYSTEM 477.875

In system tablespace ,the max size segment is IDL_UB1$,it is 167M

SQL> SELECT SEGMENT_NAME, SX || 'M' "Size" FROM (SELECT SEGMENT_NAME,SUM(BYTES)/1024/1024 SX FROM DBA_SEGMENTS WHERE TABLESPACE_NAME = 'SYSTEM' GROUP BY SEGMENT_NAME) WHERE SX > 100 ORDER BY SX DESC;
SEGMENT_NAME Size
------------- -----
IDL_UB1$ 167M

QUESTION: what's the table IDL_UB1$ used for ?