There have three major considerations when we need choose high advanced energy saving glazing: The light Transmittance (TL), the U- value, and Shading Coefficient (SC). 

The light transmittance is the ratio of the transmitted light flux to the incident light flux. Usually, the higher TL, the more natural day light.

Heat transmittance through a surface by conduction, convection and radiation is expressed by its U-value.  Usually, the lower the U- value, the lower the heat loss. 

The solar factor of glazing is the percentage of the total solar radiant heat energy entering the room.  Usually, the lower the SC, the less solar radiant heat energy entering the room through the glass.