National-level, f you worked for the government (military, healthcare, PCC) you weren't allowed to have more than one child without loosing your job (if you were grunt-level) or paying and having your career path stilled (if you were CO-level).

For other workers, it was more or less a "state-level" implementation with governors deciding how to enforce the rule. In most places, farmers had the right to have two children and could ask to have one more. City workers only one and had to pay different amount depending on the city and the governor for the second. Some governors were really, really hard on enforcement (forced abortion...) while other were more lax and made people pay acceptable amount for the second child.

Still, even without hard enforcement, the illegal child have no paper, cannot join legal school and isn't present on the family register (and can't work/live anywhere legally). This is one of the 3 internal problem the Chinese government will have with the generation.