One of the more interesting aspects of Crusader King 3 is how landholding works. Your Realm is everything, including land and titles, that you own, including the areas that are held by your Vassals.
Your Domain, on the other hand, is the total of your personally own Holdings. The Domain Limit is the maximum number of holdings that you can hold yourself, and this is impacted by a few factors.
The Stewardship Skill improves your ability to rule and manage your Domains. It will increase your income from taxes and is definitely something you should be working on with every character. You can gain one Domain Limit point for every 5 points in the Stewardship Lifestyle.
Success Law dictates what happens to Titles when you die. This can be very difficult to change in the game and will require a lot of Crown Authority. At the start of the game, your lands will be divided among any eligible children, and the game will automatically create titles for them based on their inheritance. The oldest child will normally get the best portion of the kingdom.
Your Primary Title is the highest-ranked title and determines your coat of arms and the name of your realm. The higher this is, the better.
If you own too many holdings this can impact people’s opinion of you, and your income through taxes, as you are seen to be greedy. If you are approaching your Domain Limit, you can use titles to turn other characters into your Vassals by clicking on them and selecting Grant Title.
This means they can then look after some of your holdings, allowing you to maintain control and income without impacting your Domain Limit. It is very important to keep your Vassals thinking well of you, as they can try to overthrow you, or just murder you in your sleep. So, correctly assigning land and titles will make them very happy, and you can still benefit from the land through taxes and levies.