Researchers ultimately want the answer to a research question to pertain to the

Researchers ultimately want the answer to pertain to the target population, the whole group they wish to describe.

It is the population to which findings are meant to be generalised.

The sample is only the smaller group actually studied, not the final target of the conclusion.

The accessible population is the reachable portion from which the sample is drawn, narrower than the target.

The whole world is far broader than any study's defined population of interest.

Findings generalise best when the accessible population closely matches the target population.