One more complex example:

create table t10 (a int, b int);
insert into t10 select seq, seq from seq_1_to_10;
create table t20 (a int, b int);
insert into t20 select seq, seq from seq_1_to_10;
create table t21 (a int, b int);
insert into t21 select seq, seq from seq_1_to_10;

create table t30 (a int, b int);
insert into t30 select seq, seq from seq_1_to_10;

explain
select 
  * 
from t10
where
   t10.a < 3 or
   t10.b IN (select 
               (t10.a+t20.a+t21.a) as HANDLE_ME
             from
               t20, t21
             group by HANDLE_ME
             having 1 > (select max(t30.a)
                         from t30
                         where 
                           HANDLE_ME < t30.b)
            )

Here "HANDLE_ME < t30.b" in subquery#3 is a reference to an item in subquery#2.
However, HANDLE_ME in subquery #2 is defined as 

  (t10.a+t20.a+t21.a) as HANDLE_ME

and also has a reference out of subquery#2 to subquery #1.