How long does one have to keep Hong Kong landing slips?

How long does one have to keep Hong Kong landing slips?

I understand that I have to keep a Hong Kong landing slip till I leave Hong Kong, but can I throw it away right after I leave Hong Kong? If not, in which cases do I need it? For example, the Chinese immigration asks for the Hong Kong landing slip when applying for the Shenzen SEZ Tourism Visa when entering Shenzen. I wonder whether there exist other cases when one may need the Hong Kong landing slip after leaving Hong Kong.


Hong Kong landing slip:

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