Visited countries on visa application

Many visa application forms have an entry field for "which countries have you visited before?". Being a Schengen citizen and resident with parents that also love to travel, I have visited a couple dozen countries, some I don't even remember going. None of these have border controls, except for the UK, and in practice it's one country.

Now, the form asks for all visited countries. What should I enter here (in general)? All countries or is "Schengen" okay? Or in my case, have you visited any other countries than your home country, the answer would be UK and US? I am looking for a general answer here, not just helping me!

Does the applicant's nationality make a difference? For example only list countries that do not have free movement? Does it matter much if I forgot to enter a country because I don't remember going there? If it is okay to leave Schengen countries out as a Schengen national, what about the UK? Or does it entirely depend on the country issuing the visa?

As I understand it these countries are mostly looking for visits to enemy nations, eg US - Syria.