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.

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 select this option, the import wizard will detect existing records in Salesforce that have the same external ID. Note that this operation is not case-sensitive - for example, “ABC” will be matched with “abc”. However, there is an exception: if the custom field has the separate “Unique” attribute and the case-sensitive option for that attribute is selected, uppercase and lowercase letters will not be considered identical.

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