  The forward modelling of galaxy surveys has recently gathered interest as one
of the primary methods to achieve the required precision on the estimate of the
redshift distributions for stage IV surveys. One of the key aspects of forward
modelling a galaxy survey is the connection between the physical properties
drawn from a galaxy population model and the intrinsic SEDs, achieved through
SPS codes (e.g. FSPS). However, SPS requires a large number of detailed
assumptions on the constituents of galaxies, for which the model choice or
parameter values are currently uncertain. In this work, we perform a
sensitivity study of the impact that the variations of the SED modelling
choices have on the mean and scatter of the tomographic galaxy redshift
distributions. We assumed the Prospector beta model as the fiducial input
galaxy population model and used its SPS parameters to build 9 bands magnitudes
of a fiducial sample of galaxies. We then built samples of galaxy magnitudes by
varying one SED modelling choice at a time. We modelled the colour redshift
relation of these galaxy samples using the SOM approach. We placed galaxies in
the SOM cells according to their observed frame colours and used their cell
assignment to build colour selected tomographic bins. Finally, we compared each
variant's binned redshift distributions against the estimates obtained for the
fiducial model. We find that the SED components related to the IMF, AGNs, gas
physics, and the attenuation law substantially bias the mean and the scatter of
the tomographic redshift distributions with respect to those estimated with the
fiducial model. Regardless of the applied stellar mass function based
re-weighting strategy, the bias in the mean and the scatter of the tomographic
redshift distributions are greater than the precision requirements set by
next-generation Stage IV galaxy surveys, such as LSST and Euclid.
