You use the POWERMULTISET_BY_CARDINALITY operator to get a sub-nested table from an input nested table with a specified length (cardinality).
SQL>
SQL> CREATE Or Replace TYPE nestedTableType IS TABLE OF VARCHAR2(10)
2 /
SQL>
SQL> SELECT *
2 FROM TABLE(POWERMULTISET_BY_CARDINALITY(nestedTableType('This', 'is', 'a', 'test'), 3));
COLUMN_VALUE
---------------------------------------
nestedTableType('This', 'is', 'a')
nestedTableType('This', 'is', 'test')
nestedTableType('This', 'a', 'test')
nestedTableType('is', 'a', 'test')
SQL>
SQL> drop type nestedTableType
2 /
drop type nestedTableType
SQL>
SQL>You cannot use POWERMULTISET_BY_CARDINALITY in PL/SQL.