$generation

This is the generation rate of each region. unit $cm^{-3}s^{-1}$.

Example: 
$generation 
5.0e+021 
5.0e+021 
0 

This means that only the first two regions consider the generation rate which is 5.0e21 $cm^{-3}s^{-1}$. and there is no generation in the third layer.