Calibration is defined as "an activity which, under specified conditions, establishes a relationship between the quantity values provided by standards with their measurement uncertainties and the corresponding indications with their associated measurement uncertainties and, in a second step, a relationship by means of which a measurement result is obtained from an indication" (International Dictionary of Metrology). Only calibration by an accredited laboratory can provide and verify this required metrological proof with international acceptance.
Traceable calibrations are performed only by laboratories that are accredited according to ISO/IEC 17025. This globally recognized standard defines the requirements for the competence of test- and calibration laboratories.