Related Questions:
Is the BK number a unique ID?
What can I use as a unique ID from the API?
Answer:
If you are using the API to return data on your Bookings or Events you will have two options for id's in the returned fields:
id - This is a unique identifier for the Booking and is unique across all Bookings in Event Temple, this is the same for Events.
unique_id - This is the BK number that you see in Event Temple. This number is only unique to the organization that the Booking is in and will be repeated across organizations. This is the equivalent to an EV number for an Event.