Northern Trail Outfitters (NTO) plans on sending SMS and push notifications together with emails as...

When a journey spans multiple channels — email, SMS, and push — the Contact Key must serve as a single, stable, channel-agnostic identifier tying together all of a person ' s separate channel-specific addresses (email address, mobile number, device/app identifier), because none of those individual channel addresses is guaranteed to be unique, permanent, or even present for every contact at all times. A generated, non-changing identifier such as a GUID is the documented Salesforce best practice for Contact Key precisely because it remains constant even if a subscriber ' s email address, phone number, or device changes over time, ensuring the same underlying individual is consistently recognized and correctly tracked across all channels throughout the onboarding journey without risk of fragmenting into multiple disconnected records. Using Email Address as Contact Key (B) or a phone number field (C) ties the core identifier to a single, mutable channel value that may not exist for every contact (a contact may lack a phone number, breaking SMS-based identification) and can change over time, causing tracking and deduplication problems in exactly the kind of multi-channel journey described. A shared field like Physical Address (D) is neither guaranteed unique per individual nor a standard identifier practice. Reference: Contact Key Best Practices, Multi-Channel Journeys.