SET is a collection type in which each element has different values. Elements of a SET can have many different data types or even instances of different classes.
CREATE TABLE set_tbl ( col_1 set(int, CHAR(1)));
INSERT INTO set_tbl VALUES ({3,3,3,2,2,1,0,'c','c','c','b','b','a'});
INSERT INTO set_tbl VALUES ({NULL});
INSERT INTO set_tbl VALUES ({''});
SELECT * FROM set_tbl;
col_1
======================
{0, 1, 2, 3, 'a', 'b', 'c'}
{NULL}
{' '}
SELECT CAST(col_1 AS MULTISET), CAST(col_1 AS LIST) FROM set_tbl;
cast(col_1 as multiset) cast(col_1 as sequence)
============================================
{0, 1, 2, 3, 'a', 'b', 'c'} {0, 1, 2, 3, 'a', 'b', 'c'}
{NULL} {NULL}
{' '} {' '}
INSERT INTO set_tbl VALUES ('');
ERROR: Cannot coerce '' to type set.