SMS Topics
Per-category subscription preferences, tracked separately from email.
SMS topics work like email topics — contacts opt in and out per category — but the preferences are tracked per channel: "Product updates" can be on for email and off for SMS for the same contact. A campaign tied to a topic skips recipients who opted out of it. New to the channel? Start with SMS vs email.
STOP always wins on top: a recipient who replies STOP is suppressed on the SMS channel entirely, regardless of topic preferences — and it never touches their email subscription.