What Sub-Zero refers to as "Over-and-Under" is a fridge with a bottom freezer
Over-and-Under Refrigerator & Freezer Repair
Sub-Zero over-and-under refrigerators-freezers is a broad category of appliances. These appliances have at least two separate compartments with separate sealed systems that allow greater control over the temperature settings in both the freezer and the refrigeration sections.
Higher complexity makes this configuration more prone to occasional malfunctioning, but due to the high quality of Sub-Zero appliances, average customers rarely get any bad experience with these refrigerators.
It is important to keep in mind, however, that repairing over-and-under refrigerators & freezers requires experience and professionalism.