  Mass-superselection rule (MSR) states that in the non-relativistic quantum
theory superpositions of states with different masses are unphysical. While MSR
features even in textbooks, its validity, physical content and consequences
remain debated. Its original derivation is known to be inconsistent, while a
consistent approach does not yield a superselection rule. Yet, superpositions
of masses do not seem to be present in Newtonian physics. Here we offer a
resolution of the MSR puzzle. Crucial for the result is the understanding of
two issues: how the notion of a mass parameter arises from the fundamentally
dynamical relativistic notion of mass-energy, and what is the correct Newtonian
limit of relativistic dynamics of composite particles. The result explains the
physical content of the MSR without invoking any non-standard physics and
clarifies the relation between MSR and a formalism describing relativistic
composite particles, developed for studying relativistic effects in table top
experiments.
