Universal Containers has an order system that uses an Order Number to identify an order for customers and service agents. Order records will be imported into Salesforce. How should the Order Number field be defined in Salesforce?

To maintain a unique identifier from an external system, the field in Salesforce should be marked as an External ID. This attribute signals to the Salesforce platform that the field can be used by the API or data import tools (like the Data Loader) to match records during data operations, particularly for upsert calls. Making the field Unique enforces data integrity by preventing duplicate Order Numbers from being created within Salesforce, which is essential for a primary identifier.

A. Indirect Lookup: An indirect lookup is used on an external object to link it to a standard or custom object in Salesforce. The question describes importing data into a Salesforce object.
B. Direct Lookup: "Direct Lookup" is not a standard Salesforce field type or attribute. A standard lookup creates a relationship between two Salesforce objects, which is not what is being defined here.
D. Lookup: A lookup field creates a parent-child relationship between two records within Salesforce. The Order Number is a value from an external system, not a reference to another Salesforce record.

1. Salesforce Developer Documentation - Custom Field Attributes:

External ID: "This option makes the custom field searchable in the sidebar and available for use by the API and external applications. For custom objects and solutions, you can use a custom field of this type as an external ID. When you select this option, you can also select whether the field is Unique, in which case the values in the field must be unique, and whether it is required."

Reference: Salesforce Help, Custom Field Attributes, Section: "External ID".

2. Salesforce Developer Documentation - upsert DML Operation:

The upsert operation uses a specified field to determine whether to create a new record or update an existing one. This field must be designated as an External ID. "The upsert statement matches the sObjects with existing records by comparing values of one field. If you don't specify a field, the upsert statement uses the sObject’s ID to match the sObject with existing records in Salesforce."

Reference: Apex Developer Guide, Chapter: "Database DML Operations", Section: "Upserting Records".

3. Trailhead - Data Modeling Module:

"An external ID is a custom field that has the External ID attribute, meaning that it contains unique record identifiers from a system outside of Salesforce... When you import custom objects, solutions, or person accounts, you can use external IDs to prevent duplicates."

Reference: Trailhead by Salesforce, Module: "Data Modeling", Unit: "Create Custom Fields".