https://www.e-rt2012.fr/coefficients-caracteristant-une-fene...
"Le facteur solaire, défini par les coefficients Sw ou FS, est un nombre sans unité qui définit la capacité de votre fenêtre à transmettre la chaleur d’origine solaire à l’intérieur de votre local. Ainsi, plus le coefficient Sw est élevé, plus votre fenêtre laissera passer l’énergie solaire."

Also called the G-value.

https://en.wikipedia.org/wiki/Solar_gain#G-Value
"G-value (sometimes also called a Solar Factor or Total Solar Energy Transmittance) is the coefficient commonly used in Europe to measure the solar energy transmittance of windows. Despite having minor differences in modeling standards compared to the SHGC, the two values are effectively the same. A g-value of 1.0 represents full transmittance of all solar radiation while 0.0 represents a window with no solar energy transmittance. In practice though, most g-values will range between 0.2 and 0.7, with solar control glazing having a g-value of less than 0.5."