The requirement explicitly calls for an attribute-to-attribute comparison between the customer number field in the journey ' s entry source data extension and the customer number field in a separate transaction data extension. A Decision Split configured for ' both Contact and Journey Data ' allows the split ' s filter criteria to reference attributes from Attribute Groups configured in Contact Builder (relational, contact-scoped data such as the transaction/redemption DE) alongside the journey ' s own entry-source attributes in a single condition — precisely the cross-DE, field-to-field comparison described. A split using only Journey Data (A) is confined to the fields present in the entry source itself and cannot reach into the separate transactional data extension. A split using only Contact Data (B) can query related, contact-linked data but loses the ability to reference journey-specific attributes in the same condition, which the scenario also implies is needed for the comparison. Einstein Split (C) is a machine-learning-driven random/predictive path allocator, not a rules-based attribute comparison tool, and is unrelated to this use case. Consultants should recognize ' compare a field in the entry source against a field in a related data extension ' as the signature use case for the combined Contact-and-Journey Data Decision Split. Reference: Journey Builder Decision Split, Data Designer Relationships, Attribute Groups.