Can you please elaborate on how the <generator> will be used for composite PK?. In my situation XYZ_REQUEST_ID needs to be auto-generated from xyz.sequence_application_id sequence, and XYZ_REQUEST_SITE_ID is a foreign key .
Basically you define a generator to use and Hibernate will pick up a value for a primary key from that. In your case you have a sequence to use, so the generator will just get the next value form there. Have a read of the documentation for a fairly detailed explanation.