Crusader Kings 3 has an Ironman Mode that prevents regular progression. Players used to be able to switch characters by quitting the game, loading it again, and choosing someone else from the map, but Iron Man comes with its own set of challenges.
Ironman Mode is an optional way to play Crusader Kings 3 that adds a lot more difficulty by removing control over save files. It prevents the player from correcting mistakes and changing their decisions by only providing a singular save file. You cannot have more than one save on a character. This way, you're prevented from redoing anything.
Can I switch characters if I use Ironman Mode in Crusader Kings 3?
No, you cannot switch characters if you use Ironman Mode. This would make the game much easier for players, as they could switch characters within their dynasty to accomplish their objectives.
Can I earn achievements in Crusader Kings 3 without Ironman mode?
Ironman Mode is the only way to earn achievements in Crusader Kings 3. Mods and console commands are allowed in Crusader Kings 3, so cheating is quite easy, and that would make achievements unfair. The Ironman Mode does not allow for any alterations, so it is a much more fair mode to earn achievements in.
For more on Crusader Kings 3, check out What year does Crusader Kings 3 end? or What is Cultural Acceptance in Crusader Kings 3: Royal Court and how to increase it here on Pro Game Guides!