* An External ID field is used to match data in Salesforce with data from external systems (like an on- premise SQL database). It allows for easy upserts and data integration without relying on Salesforce IDs.
Why not other options?
* A: External objects are used for real-time integration with external systems but do not address data matching directly.
* C: Formula fields are calculated fields and cannot be used for data matching.
* D: Lookup fields are used for creating relationships between objects within Salesforce, not for matching external data.