Marketing Cloud Connect requires a Subscriber Key that matches a Lead ID, Contact ID, or User ID for native tracking and send logging to work properly. If a custom object is used without these IDs, Marketing Cloud cannot push tracking data (such as email opens or clicks) back into Sales Cloud against those recipients.

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Exact Extract from Salesforce Documentation:

Source: Salesforce Help – Marketing Cloud Connect Tracking

→ "Tracking data is automatically pushed back to the appropriate Lead, Contact, or User object when using Salesforce Send Logging. If the subscriber key does not match these standard IDs, tracking data cannot be associated with recipients in Sales Cloud."

Source: Salesforce Marketing Cloud Connect Guide – Contact Model Requirements

→ "A standard object ID (Lead ID, Contact ID, or User ID) must be used to ensure synchronization and tracking between Salesforce and Marketing Cloud."

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