By default this file contains only comments, but you can put country level modifiers that are applied under certain conditions here. The conditions are checked once per month. For example:
populationsize_30k = { trigger = { total_pops = 7500 NOT = { total_pops = 25000 } } research_points = -1 global_population_growth = 0.003 icon = 11 }
The trigger part contains the conditions, the icon is one from the modifier icons and the rest of the triggered modifier contains the country scope modifier effects that have to be applied.