Universal Containers has a new custom object for Invoices that includes an Invoice Number field.

A Text field configured as External ID and Unique satisfies both requirements. Existing Invoice Numbers can be migrated directly from the external system without changing their values, while the Unique attribute prevents duplicate Invoice Numbers from being stored later. Salesforce defines an External ID as a field containing an identifier from a system outside Salesforce and supports Text fields as External IDs. Salesforce also states that marking the field Unique prevents duplicate values and makes it suitable for record matching and future data operations. An Auto Number field would generate Salesforce-controlled values and therefore would not preserve externally assigned Invoice Numbers. Maintaining separate legacy and Salesforce numbering fields is unnecessary when one field can preserve the external identifier and enforce uniqueness. Salesforce

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