Migration balancing makes sure that all parts of population change add up. In the cohort-component method, out-migration is not set on its own. The system works out how many people must leave so that births, deaths, in-migration, and out-migration match the total.
The equation is:
Out-Migration = Population(t−1) + Births − Deaths + In-Migration − Population(t)
This keeps the projection consistent. Every person is counted once.