Are CRM certificates considered external documents?

Yes, CRM certificates are external records. Any information that is required by the laboratory, but not created by the laboratory, is an external document.
Note that they require correct control as the requirements of clause 6.4, Equipment, apply to CRM certificates too. This means that the reference data needs to be verified as fit for use, providing sufficient information to maintain confidence in reference material. The storage and handling of the Reference data, including results, acceptance criteria, relevant dates, and the period of validity must also be controlled.