That's right; because the population digit gives you such a broad value (Pop Digit 6 represents 10^6 people, or somewhere from 1 million to 9 million people), the population multiplier makes it just a bit more fine-grained.
So for example, using a Population Digit (Pop) of 7, with a variety of Population Multipliers (PM):
Pop 7, no PM: 10,000,000 to 90,000,000 people
Pop 7, PM 1: 10,000,000 people
Pop 7, PM 5: 50,000,000 people
Pop 7, PM 9: 90,000,000 people