Use an affiliate audience filter when an email should go only to attendees referred by a particular affiliate, or when you want to exclude that affiliate's referrals.
Add the affiliate filter
Go to Email & SMS > Emails.
Open the Campaigns tab and edit the manual or automated email.
Open Audience and make sure the audience is Attendees.
Under Audience Filters, select Add New Filter.
Choose Marketing Source, then Affiliate.
Choose Is Equal To to include referrals from that affiliate, or Is Not Equal To to exclude them.
Select the affiliate and save the email.
Availability and limits
Affiliate choices come from affiliates linked to the event. Audience rules cannot be changed for transactional emails, follow-up emails, or an email that has already been sent. Manual and automated emails are plan-dependent; if the email shows Upgrade Required, the feature is unavailable on the event's current plan.