IS A SET operator checks whether a variable is a VARRAY or NESTED TABLE collection variable
SQL>
SQL> DECLARE
2 TYPE list IS TABLE OF INTEGER;
3 a LIST := list();
4 BEGIN
5 IF a IS A SET THEN
6 dbms_output.put_line('"a" is a set.');
7 END IF;
8 END;
9 /
"a" is a set.