The evaporator removes heat because the refrigerant inside it is colder than the room air. This difference is the TD: with a 0 °C room and −8 °C evaporating, the TD is 8 K. As the TD grows the same evaporator gives more capacity, but because its surface is colder it pulls more moisture from the air and the room dries out. In a catalog, evaporator capacity is always given for a specific TD; most European catalogs use 8 K, some 6 or 10 K.
Product humidity sets the TD: in fruit, vegetable and flower rooms wanting humidity above 90%, TD 4–6 K; in meat and general storage rooms wanting 85–90%, 6–8 K; in packaged product and frozen stores where humidity does not matter, 8–10 K. A small TD means a larger (more expensive) evaporator; the return is less product weight loss.
TD also affects system efficiency: for every 1 K drop in evaporating temperature, compressor capacity falls by about 3% and electricity consumption rises. A small evaporator chosen with an unnecessarily large TD looks cheap at purchase but costs more in operation. The capacity tables in the evaporator catalog let you compare models at different TD values.