Shading coefficient (SC) is an alternative measure of the heat gain through glass from solar radiation. SC is an older term being replaced by solar heat gain coefficient (SHGC).
Specifically, the shading coefficient is the ratio between the solar heat gain for a particular type of glass and that of double-strength clear glass.
A lower shading coefficient indicates lower solar heat gain. For reference, 1/8" (3.1 mm) clear glass has a value of 1.00.