In what scenario should the Use as Root checkbox be marked when joining a data extension in Contact Builder?

The "Use as Root" checkbox should be marked in scenarios where the data extension serves as the primary reference for contact records in your marketing efforts. Specifically, when you have a database of record other than All Subscribers, marking this checkbox indicates that this data extension will be the foundational source of truth for individual contacts, overriding the default All Subscribers list.

In the context of Contact Builder, designating a data extension as the root enables marketers to create more robust and specific contact models. It signals that this particular data extension holds essential information that is critical for resolving and segmenting contacts effectively.

While other options present valid scenarios for joining data extensions, they do not necessitate the root designation. For instance, supplemental data for an interaction, relationships in a one-to-many context, or data for both email and MobileConnect sends do not require the same level of primary importance within Contact Builder's framework. These scenarios can rely on standard linkages without needing a primary designation, as they might simply serve to enhance existing contact data without fundamentally redefining it.