In large holdover plate refrigerator and freezer systems, liquid refrigerant can condense in the plate evaporator lines when the compressor is not running. On startup, this liquid is suddenly dumped into the compressor causing liquid "slugging" which can cause damage. Installing a suction accumulator in the suction line protects the compressor from this liquid slugging by ensuring that only gas is returned compressor.
The term "sub-cooling" refers to the process of further cooling the liquid refrigerant (after it exits the condenser) to a temperature below its "dew point (i.e the temperature at which the gas condenses into a liquid). The result is a substantial improvement in energy efficiency and system capacity