Found this in Spinward Marches Campaign, just in case this question comes up in a search again:
The three digits indicate successively the population level, the number of planetoid belts in the system, and the number of gas giants in the system. Population Level: This number times 10 raised to the Population power given in the UPP, indicates the population of the world. For example, world 0236 (695735-9) has a population level of 6. Its population is 6x10^7.
SMC pg 22
The three digits indicate successively the population level, the number of planetoid belts in the system, and the number of gas giants in the system. Population Level: This number times 10 raised to the Population power given in the UPP, indicates the population of the world. For example, world 0236 (695735-9) has a population level of 6. Its population is 6x10^7.
SMC pg 22
