I am learning for my 401 dev exam and reading the book For.com Developer Certification Handbook DEV401. This book is written a little weird and sometimes is even contradictory to itself. therefore I am not certain that all information is correct. At some point, it says:
A custom field can be marked as an external ID to identify the primary keys of the legacy system (outside Salesforce). Text, number, e-mail, and auto-number fields can be flagged as external IDs.
but this seems so weird. I mean, an auto-number is a number that is generated by Salesforce, why would you not want to keep track of one generated unique identifier (Salesforce ID) but then keep track of another unique identifier (auto-number). Does anybody know if this information is legitimate?