externalID

Great question! Building on the earlier comment: the external_id in Braze should be a unique identifier that represents an individual person across your systems. If your customer_loyalty_id only applies to loyalty members—and not to every customer or user—it’s better to keep that as a custom attribute on the user profile.

That way, you can still identify and target loyalty members, without using it as the main ID that Braze relies on to recognize users. Your external_id should be something consistent and universal for every user, not just a subset like loyalty members.