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After being subscribed, a waiting list for the Artist is appear in the system (Contact > Waiting List), operator could send alert for the contact when the Artist is being on sale.
If the option "Double opt-in" is enabled for the Ticket shop sale channel, contact will be receiving a confirmation email for the waiting list (at this stage, the state of waiting list is PENDING), and operator could only send the alert once contact has confirmed the subscription to the Artist (the state is updated to OPEN).