Powers often have effects or effect groups that are specific to PvE or PvP, yet how that's implemented is inconsistent. Some do a requirement check for isPvPMap?, some have a flag for PvPOnly?, and some check if the target is a Player.
In the case of Singularity, it uses a requirement check for if target is a critter and if target is a player. Since Singularity is a pet that can be summoned by either players or critters, that "if target is player" requirement is used for both PvP and EvP. This is why a PvEOnly flag was added to this effect. It will retain the EvP aspect of the powe