Population Ethics addresses the question: how should utilitarians deal with people coming in and out of existence?
For example, a classical problem in population ethics: is it overall better for a happy person to exist, if they bring down the average happiness but increase the total? Those who would answer "yes" are termed total (hedonic) utilitarians; those who would answer "no" are termed average (hedonic) utilitarians.
Note that utility is not comparable, so we cannot simply speak of "increasing average utility" vs "increasing total utility". Hence, average utilitarianism vs total utilitarianism is not a well-defined distinction in the abstract; we have to specify e.g. hedonic utilitarianism before this distinction becomes well-defined (because average vs total happiness is a meaningful distinction).
However, although preference-utilitarians cannot necessarily make a meaningful distinction between averaging and totaling, they do not escape population-ethics dilemmas. It's still a fair question: when is it overall preferable to bring someone into existence (or for someone to pass out of existence)? What can/should ethics say about this?