In EU3 terminology, i think more appropriate term would be modifiers.
The most basic ones are in definies.txt. They contain the "hard" information that is used to create the EU3 world.
Then you have static_modifiers.txt which contains more advanced stuff, like bonuses or penalties for different difficulty levels, stability, legitimacy, all that "soft" stuff superimposed once the world is created.
After that you have triggered modifiers (self explained).
And then you have modifiers that are assigned by events and stuff, those are located in event_modifiers.txt.
First and second group, you cannot create, you can remove (although that will almost certainly screw the game up) and you can alter. That means you can change their effects, but changing them (as in renaming etc) or removing them is not recommended.
Third group you can manipulate as you like, however it does take some skill and experience.
And fourth group you can do as you like, add, remove, modify, everything is allowed. However, note that every time you create an event (for example) that gives the triggered country a modifier, and that modifier does not exist in event_modifiers.txt, game will crash to desktop. I strongly recommend against removing any of the existing ones, unless you know EXACTLY what youre doing.