11.4.7. About Collection Member References

References to collection-valued associations actually refer to the values of that collection.

Example 11.8. Collection References Example

select c
from Customer c
    join c.orders o
    join o.lineItems l
    join l.product p
where o.status = 'pending'
  and p.status = 'backorder'

// alternate syntax
select c
from Customer c,
    in(c.orders) o,
    in(o.lineItems) l
    join l.product p
where o.status = 'pending'
  and p.status = 'backorder'
In the example, the identification variable o actually refers to the object model type Order which is the type of the elements of the Customer#orders association.
The example also shows the alternate syntax for specifying collection association joins using the IN syntax. Both forms are equivalent. Which form an application chooses to use is simply a matter of taste.