  We examine the notion of nonclassicality in terms of quasiprobability
distributions. In particular, we do not only ask if a specific quasiprobability
can be interpreted as a classical probability density, but require that
characteristic features of classical electrodynamics are resembled. We show
that the only quasiprobabilities which correctly describe the superposition
principle of classical electromagnetic fields are the $s$-parameterized
quasiprobabilities. Furthermore, the Glauber-Sudarshan P function is the only
quantum-mechanical quasiprobability which is transformed at a classical
attenuator in the same way as a classical probability distribution. This result
strengthens the definition of nonclassicality in terms of the P function, in
contrast to possible definitions in terms of other quasiprobabilities.
