Offering Indulgences to the Poor souls in Purgatory

Obviously this Question is strictly for Catholics. When all conditions are met from those being granted an indulgence, particularly a plenary indulgence for the souls in Purgatory. Is it necessary that the individual be named or can the indulgence be offered by mode of suffrage to an unknown soul suffering the punishments of forgiven sins? I have been unable to find a source for this, so by default have assumed that the indulgence can be issued to the unknown soul.