  The one-dimensional pair contact process with diffusion (PCPD), an
interacting particle system with diffusion, pair annihilation, and creation by
pairs, has defied a consensus about the universality class that it belongs to.
An argument by Hinrichsen [H. Hinrichsen, Physica A {\bf 361}, 457 (2006)]
claims that freely diffusing particles in the PCPD should play the same role as
frozen particles, when it comes to the critical behavior. Therefore, the PCPD
is claimed to have the same critical phenomena as a model with infinitely many
absorbing states that belongs to the directed percolation (DP) universality
class. To investigate if diffusing particles are really indistinguishable from
frozen particles in the sense of the renormalization group, we numerically
study a variation of the PCPD by introducing a nonorder field associated with
infinitely many absorbing states. We find that a crossover from the PCPD to the
DP occurs due to the nonorder field. Since, by studying a similar model, we
exclude the possibility that mere introduction of a nonorder field to one model
can entail a nontrivial crossover to another model in the same universality
class, we attribute the observed crossover to the difference of the
universality class of the PCPD from the DP class.
