OPM must review and decide upon an agency's request to pass over a candidate, who is a preference eligible, on medical grounds pursuant to §339.306. OPM may deny an applicant employment by reason of physical or mental unfitness for the position for which he or she has applied. An OPM decision under this section or §339.306 is separate and distinct from a determination of disability pursuant to statutory provisions for disability retirement under the Civil Service Retirement System and the Federal Employees' Retirement System.