$generation

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

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 $c{m}^{-3}{s}^{-1}$. and there is no generation in the third layer.