There are two options for managing these hybrid sessions:
Publish two identical listings for the session, one for customers to book a physical space at the venue and another to book for a virtual space on the livestream. This is the recommended process.
Alternatively, use one session listing but create two single credit bundles in PACKAGES, one to be used by customers booking a physical space at the venue and another for customer only wishing to access the livestream. Whilst you cannot cap how many of each bundle will be used at booking, you will be able to see how many of each type have been booked from the ATTENDEES tab under the Payment Method column.
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