Soldiers are the POP type, which represent the fighting men of a nation. They are used for raising armies, with 3000 soldiers being used in each brigade. However, only 1000 soldiers are needed to create the first brigade from a province. Soldiers are also used for reinforcing damaged brigades, and the soldier POP will take damage when men are killed in action. The amount of people lost can be reduced via the nation's "military hospitals" level, which can be increased by researching certain technologies and obtaining certain inventions. Brigades have the potential to revolt, and will do so whenever the POP they stem from does.
Like officers they're paid directly from the military spending slider, so if spending is too low they won't meet their needs and demote, while if spending is high promotion to the POP-type will be increased.
The following factors affect the likelihood of a Soldier pop choosing to migrate to a particular province. Notably, soldier pops are far more likely to congregate in state capitals. This is designed to increase the chance that soldier pops are large enough to support battalions.