Have you ever wondered why 2.1 billion seems to be the highest number of any item that you can have in the game? You've probably wondered why this is the cut-off if you're among the many who have had 2.1 billion marks only to have them disappear on you. 2.1 billion is a pretty odd number, after all. Why couldn't the programmers cap it off at 2 billion instead?
To be precise, 2,147,483,647 is the highest amount of any item that you can possibly have in the game. Marks are simply the most common items to have 2.1 billion of since the top 24 Guild list recognizes the people who have this many marks. But why 2,147,483,647? That's because it's the maximum value of a signed integer in Visual Basic, C++, and Java, meaning that this is the maximum amount of anything that involves numbers in the game. Exceeding this number will result in the loss of a lot of items.
Just for Kicks
What would happen if there were 2,147,483,647 characters in the game? It's possible that the game would reset or malfunction in some way, but luckily, it isn't realistic to have so many characters since there would never be enough server space to store information for all of those characters. That, and between 200,000 to 400,000 characters existed just a few years ago when the game was at its peak. Today, it seems that less than 10,000 exist. This certainly isn't something that you have to worry about.
And what would happen to a clan that was somehow able to acquire more than 2,147,483,647 power? My guess is that the clan would end up with negative power or it'd dismantle itself somehow.