luggage from HKIA flight going to Guangzhou

Hello,
I've read on your posts that luggage will be automatically transfered to the ferry from the HKIA flight arrivals. We also would do immigration things through the ferry.
I'm worried about my luggage. How do the airline baggage people know which bags to transfer to the ferry? Do I need to have a ferry ticket before I board my flight that arrives in HKIA? How would they identify my bag to be put on a ferry if I don't even know which ferry I would take nor have a ticket yet?
Thanks-