What are best practices for AllSubscribers list management and subscriber key selection?

The AllSubscribers list is the master subscriber repository in Marketing Cloud. Every contact who receives an email is added here.
Subscriber Key best practices:
— Use a persistent, unique identifier (CRM ID, customer ID) as the subscriber key rather than email address.
— Email addresses change; a CRM ID never changes — this prevents duplicate subscriber records when a customer updates their email.
— In a connected Salesforce org, use the Salesforce Contact or Lead ID as the subscriber key for seamless sync.
— The subscriber key should match the ContactKey used in Contact Builder for unified data linkage.
Subscriber statuses:
— Active: Eligible to receive emails.
— Unsubscribed: Globally opted out; no commercial sends.
— Held: Automatically placed on hold after bouncing multiple times; requires admin review to reactivate.
— Bounced: Has received at least one hard bounce.