  The centrality determination and the estimated fluctuations of number of
participant nucleons $N_{part}$ in Au-Au collisions at 1.23 $A$GeV beam kinetic
energy suffers from severe model dependencies. Comparing the Glauber Monte
Carlo (MC) and UrQMD transport models, it is shown that $N_{part}$ is a
strongly model dependant quantity. In addition, for any given centrality class,
Glauber MC and UrQMD predicts drastically different $N_{part}$ distributions.
The impact parameter $b$ and the number of charged particles $N_{ch}$ on the
other hand are much more correlated and give an almost model independent
centrality estimator. It is suggested that the total baryon number balance,
from integrated rapidity distributions, can be used instead of $N_{part}$ in
experiments. Preliminary HADES data show significant differences to both, UrQMD
simulations and STAR data in this respect.
