The Salesforce ID is the unique key assigned to all records in Salesforce.com. It is what references each record in the backend of Salesforce.

For all entity types, every record has an ID field that uniquely identifies that record. The value for the ID field is created automatically upon insert, and it cannot change over the lifetime of that record, even if the record is deleted and then undeleted. The ID of a record is the best way to uniquely identify that record.

When you´re viewing a record within Salesforce via a browser, the URL to the record is composed of the instance (it's the server(s) that your org/database resides on) and record ID.

For e.g. http://eu11.salesforce.com/0030Y00000AKQ4L, where “0030Y00000AKQ4L” (1) is the record ID and “eu11" is the instance.

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