Top answer:
With respect to the United States of America, there is not a singular exhaustive list of reasons or regulations that can lead to a country being on a designated list - there are many agencies that enforce such things, as well as orders a...
With respect to the United States of America, there is not a singular exhaustive list of reasons or regulations that can lead to a country being on a designated list - there are many agencies that enforce such things, as well as orders and guidance from the Executive branch of the government. Banking institutions also have their own compliance policies regarding countries and/or citizens they will not do business with.
Reasons would generally involve money laundering, terrorism, fraud, etc. - and what might happen to citizens of any country depends on many factors. You may find this link generally helpful:
https://www.dowjones.com/professional/risk/glossary/anti-money-laundering/importance-aml-policies/
Natacha Ralainirina