  Bipartite entanglement may be reduced if there are restrictions on allowed
local operations. We introduce the concept of a generalized superselection rule
(SSR) to describe such restrictions, and quantify the entanglement constrained
by it. We show that ensemble quantum information processing, where elements in
the ensemble are not individually addressable, is subject to the SSR associated
with the symmetric group (the group of permutations of elements). We prove that
even for an ensemble comprising many pairs of qubits, each pair described by a
pure Bell state, the entanglement per element constrained by this SSR goes to
zero for a large number of elements.
