I can't figure out why use autonumber as External ID.
There is some case when must be used an auto number as ExternalID?
Salesforce Stack Exchange is a question and answer site for Salesforce administrators, implementation experts, developers and anybody in-between. It only takes a minute to sign up.Sign up to join this community
there are many reasons. some are:
And last but not the least, its always good to have a standard indexed field as external ID, than building custom field with lots of logic in backend
One useful feature of making an external ID auto-number field is to sync up with an external system in a bi-directional data flow where the external system is not aware of, or otherwise cannot use, the Salesforce ID. There's a number of reasons why this might happen, but the point is that Salesforce is the master data system, but the external system can create and update records using a Salesforce controlled value. You can also use this as a way of "hiding" Salesforce IDs intentionally, where the external system only knows the auto-generated value, but not the actual ID.