You can replace every instance of SEP (single experience person) with person. I came up with the terminology myself. The idea is that it is simpler to theorise about multiple person utility when we ignore the fact that people have multiple experiences over their lifetime.

You may want to read about https://en.wikipedia.org/wiki/Average_and_total_utilitarianism and the repugnant conclusion: http://utilitarianism.wikia.com/wiki/Repugnant_conclusion