Explanation
Two best practices that should be utilized for configuring customer data for long-term cleanliness and maintainability are:
When using an external database of record, utilize that system's identifier as the contact key. This will ensure that contacts can be identified and tracked across different channels and platforms using a consistent and persistent value.
Define the subscriber key as a unique value that does not relate to a specific channel. This will allow contacts to change their email or mobile device without affecting their subscriber key or losing their tracking history.
Configuring Contact Builder to automatically generate a unique subscriber key is not recommended, as it may create duplicate or inconsistent records if contacts already have an identifier in another system. Regularly merging duplicate contacts is not a best practice, but rather a remedial action that should be avoided by using proper data hygiene techniques. References:
https://help.salesforce.com/s/articleView?id=sf.mc_co_subscriber_key.htm&type=5