It is a difference between both the type of material used for the coating and the way the coating is applied to the glass.
However, there are several further differences:
Soft coated Low E glasses must be incorporated into a double glazed unit as the coating is ‘less durable’ than a hard coated (pyrolytic) coating when exposed to the atmosphere. Soft coated Low E glasses cannot be glazed in single glazed applications but hard coated products such as ComfortPlus™ and EnergyTech™ can.
Pyrolytic coatings have a long shelf life, whereas soft coated Low E glasses do have a shelf life and will begin to break down if stored for too long without being incorporated into double glazed units.
Soft coat Low E glasses such as Viridian LightBridge™ or PerformaTech™ have significantly lower U values than hard coated pyrolytic glass types.
Hard-coated Low E glasses are manufactured online during the float manufacturing process where soft coats are applied by a special offline process at some time after the glass has been manufactured.