Calibrate : see where it is against a traceable standard. When i apply 1 volt from a known source : what does the machine show ?
Verify : see if it falls within allowed tolerances. If applied 1 volt and allowed 5% error (factory specs) does it fall in this window?
Adjust : bring it back inside the VERIFIED range !!! that does not mean it will be exact ! It may be exact , but if it passes VERIFICATION the job is done !