JPA when to use Path of entity type instead of Join?

An example in JPA spec:

CriteriaQuery<Customer> q = cb.createQuery(Customer.class);
Root<Customer> customer = q.from(Customer.class);
Join<Customer, Order> order = customer.join(Customer_.orders);
q.where(cb.equal(cb.treat(order.get(Order_.product), Book.class)
.get(Book_.name),
"Iliad"));
q.select(customer);

order.product is a path in the where clause. If it is not join, how to access
the name attribute of the product(Book)? Is it actually a table join when translated to SQL? If this is the case, what is the difference between path
and join in this example?

    Order        Product
----------      -------------
    productId    id     name

JPA when to use Path of entity type instead of Join?