its A.

🧠 Why This Strategy Covers Both Requirements:
Universal Containers has two distinct needs:

Preserve legacy Invoice Numbers during migration from the external system

Assign unique identifiers to every new Invoice created in Salesforce

Here's how to address both:

Text Field for Legacy Invoice Numbers

Use this to store the Invoice Numbers from the external system.

Populate it during data migration using a tool like Data Loader or a deployment script.

Mark it as Unique if needed to prevent duplicates during import.

AutoNumber Field for New Invoices

Automatically generates unique identifiers for any new record created within Salesforce after the migration.

Ensures consistency and avoids manual entry errors going forward.

🔄 Keeping both fields allows full traceability and supports any historical audits or cross-system reconciliations while future-proofing operations with Salesforce-native numbering.