In the Send Relationship section of a data extension, the Subscriber ID should be used as it links the data extension to the subscribers within Salesforce Marketing Cloud. This ensures that the email is sent to the correct subscriber by matching the Subscriber ID with the unique identifier in the Marketing Cloud subscriber list.

Importance of Subscriber ID: This ID is essential for tracking and linking interactions, and ensuring that each email is accurately associated with the correct recipient within the journey.

Salesforce Documentation Reference: For more information on Send Relationships, see Setting Up Sendable Data Extensions.