A BSUID has two defining properties: it is unique per user, and it is scoped to the business. The same customer gets a different BSUID with every business portfolio, so identifiers cannot be cross-referenced between companies.
The identifier arrives in the user_id field of webhooks. BSUIDs started appearing in all webhook responses on March 31, 2026, and WhatsApp's Contact Book began automatically mapping existing phone-number relationships in April 2026, which is why established opt-in lists survive the transition.
Systems that only ever stored phone numbers must learn to read and store BSUIDs, and use the Request Contact Info flow for the rare cases where a real number is required. Platforms like Kanal handle this identifier plumbing, so campaigns and automations keep working either way.
