The PCO medical examination also known as a TPH 204, makes sure you are medically fit to drive passengers. As a large amount of a driver’s time would be spent behind the wheel, TfL need to ensure that a PCO licensed driver is medically fit to do so through the PCO medical. TfL use standards they specify for a DVLA Group 2 licence (Large good vehicles and buses) which are notably higher because there must be a specific set of rules for drivers that are employed behind a wheel.