independent = yes/no
What tiers can make the title? (i.e. everyone, everyone below emperor or everyone below king)
Code:NOT = { OR = { tier = KING tier = EMPEROR } }
This would prevent kings and emperors from making the titles. I use this in my titular title mod in order to keep the UI clean for players as they probably don't want to see a bunch of kingdoms they don't care about once they are king. Technically also a lockout for the AI kings and emperors, but I've never seen them willingly make them once they are a king/emperor anyways.
Whether a character has to be independent to create a title depends on the titular title system that Paradox setup. As for preventing kings and emperors making the title, I have it set so that you can only create a title if you have the lower tier title. For example, you can only create the Kingdom of York if the Duchy of York is your primary title. Likewise, you can only create the Empire of York if you have the Kingdom of York as your primary title.