Hibernate version 2.1.2

How may I map attributes in a many to many relationship?

create table REFERENCE_ENTITY
(
id integer IDENTITY primary key,
name varchar,
);
create table POOL
(
id integer IDENTITY primary key,
name varchar,
)

create table POOL_SET (
poolId integer not null,
referenceEntityId integer not null,
anAttributeOfMyrelations varchar,
entityIndex integer not null,
foreign key (poolId) references POOL(id),
foreign key (referenceEntityId) references REFERENCE_ENTITY(id)
);

I have 2 classes, one for Entity and one for Pool and the usual many-to-many realation implemented as a list.
The question is:
how do I model my anAttributeOfMyrelations ?

Many thanks!!!!

I understand that the solution is in section 7.2 of the manual. I was wondering if anyone got some examples I could start from....

Many thanks!!!!