What Is External ID In Salesforce?
External ID in Salesforce is any custom field that has the “External ID” attribute checked meaning that it contains unique record identifiers from a system outside of Salesforce.
This operation is case-insensitive but if the custom field has a separate “Unique” attribute then the case-sensitive option for that field is selected which means Uppercase and Lowercase letters will not be considered identical. For example, “Abc” and “abc” will be treated as different.
An object can have at most 25 external id and Unique fields. External ID field type can only be any one of auto-number, email, number, or text.
When we select this option the import wizard will detect existing records in Salesforce that have the same External Identification.
Custom fields marked as unique also count against an object’s limit of 25 external ID fields.
Note: We should choose both “Unique” and “ExternalId” attributes, but it is not mandatory to choose the required attribute to make the field accessible on the data import wizard.