  Systems with absorbing (trapped) states may exhibit a nonequilibrium phase
transition from a noise-free inactive phase into an ever-lasting active phase.
We briefly review the absorbing critical phenomena and universality classes,
and discuss over the controversial issues on the pair contact process with
diffusion (PCPD). Two different approaches are proposed to clarify its
universality issue, which unveil strong evidences that the PCPD belongs to a
new universality class other than the directed percolation class.
