The first point to understand about refrigeration theory is that heat is energy, and it can be made to move. If enough heat is removed from a glass of water, the water will freeze to ice. When that heat is allowed to move back into the ice, the ice will melt.
An iced-up evaporator is a common service call for any service technician working in this industry. Generally, determining this is the cause of the system problem is rather easy. But what caused the evaporator to ice up may not always be obvious.
A well-designed medium temperature refrigeration system will maintain both a proper case temperature and relative humidity (rh). The preservation of food depends not only on the temperature, but also on the relative humidity within a case.
Actually, frost on the suction line only indicates that at the location where the frost is present, the suction line piping is below the dew point temperature of the surrounding ambient air and at or below 32°F. That’s it. It is neither an indication of a properly operating system nor a system defect.