Salesforce Advanced Administrator Certification Practice Test

An encrypted custom field is not permitted to be set as an External ID due to the way Salesforce handles encryption. External IDs are designed to be used in external data integrations and are often part of workflows that may require data matching or lookups where transparency and easy readability of the field value are necessary.

Encrypted fields are intended to protect sensitive data by restricting visibility, which leads to complications with their use in integrations, especially where keys or access to sensitive information is not practical or secure. Because External IDs must be accessible for lookups to function properly in integration scenarios, the restriction against encrypted fields is in place to ensure the effective operation of these processes. Therefore, the answer highlighting this limitation is accurate.