AnExternal ID fieldis 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.