You use the POWERMULTISET operator to get a sub-nested table from an input nested table. The following query illustrates the use of POWERMULTISET:
SQL>
SQL> CREATE Or Replace TYPE nestedTableType IS TABLE OF VARCHAR2(10)
2 /
SQL>
SQL> SELECT *
2 FROM TABLE(POWERMULTISET(nestedTableType('This', 'is', 'a', 'test')));
COLUMN_VALUE
---------------------------------------------
nestedTableType('This')
nestedTableType('is')
nestedTableType('This', 'is')
nestedTableType('a')
nestedTableType('This', 'a')
nestedTableType('is', 'a')
nestedTableType('This', 'is', 'a')
nestedTableType('test')
nestedTableType('This', 'test')
nestedTableType('is', 'test')
nestedTableType('This', 'is', 'test')
nestedTableType('a', 'test')
nestedTableType('This', 'a', 'test')
nestedTableType('is', 'a', 'test')
nestedTableType('This', 'is', 'a', 'test')
15 rows selected.
SQL>
SQL> drop type nestedTableType;
You cannot use POWERMULTISET in PL/SQL.