oracle 11G 查询占用空间比较大的表，为何会有$结尾的表名字

oracle 11G 查询占用空间比较大的表，为什么会有$结尾的表名字？
查询使用空间比较大的前15个表
select * from (select segment_name,bytes from dba_segments where owner = USER order by bytes desc ) where rownum <= 30

 SQL> select a.*
  2  from(
  3    select owner,segment_name,SEGMENT_TYPE, round(bytes/1024/1024/1024,2) all_size
  4    from dba_segments
  5    where SEGMENT_TYPE='TABLE'
  6    order by  all_size desc
  7   ) a where rownum <15;
OWNER                          SEGMENT_NAME                                                                     SEGMENT_TYPE         ALL_SIZE
------------------------------ -------------------------------------------------------------------------------- ------------------ ----------
SYS                            AUD$                                                                             TABLE                    2.25
POWERDESK                      OA_EMAIL_BODY                                                                    TABLE                    1.85
POWERDESK                      OA_EMAIL                                                                         TABLE                    1.38
PLAS                           PLAS_LOGIN_LOG                                                                   TABLE                    0.65
POWERDESK                      RES_APPROVE_CONTENT                                                              TABLE                    0.58
SYS                            WRI$_ADV_SQLT_PLANS                                                              TABLE                    0.42
POWERDESK                      APP_ATTACH_FILE                                                                  TABLE                    0.35