The Differences Between CAT And TAB Table

1. What's the differences between CAT and TAB?

2. Why doesn't TAB exist in dict?

3. What the appropriate situation for CAT and TAB?

sys@ORCL> desc cat
Name Null? Type
----------------------------------------------------- -------- ------------------------------------
TABLE_NAME NOT NULL VARCHAR2(30)
TABLE_TYPE VARCHAR2(11)

sys@ORCL> desc tab
Name Null? Type
----------------------------------------------------- -------- ------------------------------------
TNAME NOT NULL VARCHAR2(30)
TABTYPE VARCHAR2(7)
CLUSTERID NUMBER

sys@ORCL> select * from dict where table_name='CAT';

TABLE_NAME COMMENTS
---------- -----------------------------------
CAT Synonym for USER_CATALOG

sys@ORCL> select * from dict where table_name='USER_CATALOG';

TABLE_NAME COMMENTS
-------------------- -----------------------------------------------------------------
USER_CATALOG Tables, Views, Synonyms and Sequences owned by the user