  The concept of active spaces simplifies the description of interacting
quantum many-body systems by restricting to a neighbourhood of active orbitals
around the Fermi level. The respective wavefunction ansatzes which involve all
possible electron configurations of active orbitals can be characterized by the
saturation of a certain number of Pauli constraints $0 \leq n_i \leq 1$,
identifying the occupied core orbitals ($n_i=1$) and the inactive virtual
orbitals ($n_j=0$). In Part I, we generalize this crucial concept of active
spaces by referring to the generalized Pauli constraints. To be more specific,
we explain and illustrate that the saturation of any such constraint on
fermionic occupation numbers characterizes a distinctive set of active electron
configurations. A converse form of this selection rule establishes the basis
for corresponding multiconfigurational wavefunction ansatzes. In Part II, we
provide rigorous derivations of those findings. Moroever, we extend our results
to non-fermionic multipartite quantum systems, revealing that extremal
single-body information has always strong implications for the multipartite
quantum state. In that sense, our work also confirms that pinned quantum
systems define new physical entities and the presence of pinnings reflect the
existence of (possibly hidden) ground state symmetries.
