Then for this simpler option, on considers that the solar flux is distributed according the spectrum shape of a black body at a temperature of 5780 Kelvin (which is often the modelling taken for an ideal sun).
If users have a more precise idea of the solar flux distribution in the bands of interest, then they may prefer the other option in which the flux restricted to each spectral band (defined in the window "Spectral paramenters") can be specified.