  The universal behaviour of the directed percolation universality class is
well understood, both the critical scaling as well as finite size scaling. This
article focuses on the block (finite size) scaling of the order parameter and
its fluctuations, considering (sub-)blocks of linear size l in systems of
linear size L. The scaling depends on the choice of the ensemble, as only the
conditional ensemble produces the block-scaling behaviour as established in
equilibrium critical phenomena. The dependence on the ensemble can be
understood by an additional symmetry present in the unconditional ensemble. The
unconventional scaling found in the unconditional ensemble is a reminder of the
possibility that scaling functions themselves have a power-law dependence on
their arguments.
