If checked, prevents duplicate field values. For text fields, you can control whether values that are identical except for their case are considered unique. Select Treat “ABC” and “abc” as duplicate values to enforce case-insensitive uniqueness, or select Treat “ABC” and “abc” as different values to enforce case-sensitive uniqueness.

Note:

Custom fields marked as Unique count against an object’s limit of seven External ID fields. Custom indexing that occurs automatically in the background by Salesforce does not count against External ID limits.

Values will be indexed by default by Salesforce.

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