  We consider pure fermionic states with a varying number of quasiparticles and
analyze two types of reduced density operators: one is obtained via tracing out
modes, the other is obtained via tracing out particles. We demonstrate that
spectra of mode-reduced states are not identical in general and fully
characterize pure states with equispectral mode-reduced states. Such states are
related via local unitary operations with states satisfying the parity
superselection rule. Thus, valid purifications for fermionic density operators
are found. To get particle-reduced operators for a general system, we introduce
the operation $\Phi(\varrho) = \sum_i a_i \varrho a_i^{\dag}$. We conjecture
that spectra of $\Phi^p(\varrho)$ and conventional $p$-particle reduced density
matrix $\varrho_p$ coincide. Nontrivial generalized Pauli constraints are
derived for states satisfying the parity superselection rule.
