Under Loyalty you can pick:

• People with Digital Balance <=> X (Select compare value on next field) — customers whose store credit balance compares to the amount you enter in Trigger Compare Value (Lower, Lower or Equal, Equal, Greater, Greater or Equal).
• People with a birthday and People who made more than X checkouts (Select value on next field) — in the current version these two are swapped: choosing People with a birthday actually pulls people by checkout count (and ignores any birth date), while People who made more than X checkouts actually pulls people who have a birth date on file (and ignores the checkout-count field you fill in). Check the View Recipients preview before sending to either one.
• People who referred someone — customers who have made at least one referral.
• Affiliates — accounts enrolled in your affiliate program.
• Referee who never made a purchase — people who were referred but have not ordered.

These segments always appear in the picker regardless of whether you use the affiliate feature; if you don't run an affiliate program, Affiliates and Referee who never made a purchase will simply return no recipients.