Why External ID?
The Order Number is used to identify records uniquely, both in Salesforce and in external systems.
Marking it as anExternal IDensures it can be matched or referenced during data imports and integrations.
Why Unique?
Setting the field as Unique ensures that duplicate values are not allowed for the Order Number.
Why Not Other Options?
A: Indirect Lookup: Used for external object relationships, which is not applicable here.
B: Direct Lookup: Not relevant for unique field identification.
D: Lookup: Used for creating relationships, not for identifying unique fields.
References:External ID Field:https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta
/object_reference/field_types.htm