Comprehensive and Detailed Explanation:

Correct answers are B and D.

B — The subscriber key should be channel-agnostic (not tied to an email address or phone number).

D — If the external POS system is the source of truth, use its stable unique identifier as the Contact Key.

❌ Why others are wrong:

A: Contact Builder does not automatically generate subscriber keys; you must define them.

C: While merging duplicates is important, the question asks for initial best practices during setup, not ongoing maintenance.

Exact Extract from Salesforce Official Documentation:

Salesforce Help — Contact Key and Subscriber Key Best Practices:

"Use a stable, unique, non-channel-specific identifier as the Contact Key/Subscriber Key, preferably sourced from your system of record."

(Source: https://help.salesforce.com/s/articleView?id=sf.mc_es_contact_key_best_practices.htm )

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