Merge will store and create unique IDs for each object record (this is the id field in a Merge API response). The remote_id field, on the other hand, corresponds to the unique ID assigned by the third-party platform. If you are trying to match an employee or candidate from Merge to the record in a specific platform, it is best practice to do so based on the remote_id.
When trying to reconcile data between Merge and third-party systems, if the record type has changed, the remote_id will be different. In these cases, Merge recommends:
In the case of reconciling Employee objects:
Most integrations that utilize the Employee common model will contain information regarding first_name, last_name, work_email. Therefore, a combination of these fields is a good string to match on for employee records.
Fields such as employee.ssn are not recommended because the are encrypted.
In the case of reconciling Candidate objects:
Most integrations that utilize the Candidate common model will contain information regarding first_name, last_name, personal_email. Therefore, a combination of these fields is a good string to match on for candidate records.