Something's been bugging me...
 

My current Campaign is kind of winding down, and frankly the worl's gotten a bit stale, so i was gonna start a new realm and map the new one out in CC3+. When I was starting out before i even named the world I noticed something that's bugged me before, thought i'd see if anyone knows.

What is "Custom Structure" in Create Realm? I've never known what it even meant, so i've always just ignored it. I click on it and it wants me to load a program.

Custom Structure is where you have an existing RW structure file from elsewhere, which you will use for your realm instead of one of the predefined structures.

Do they even have those? How do you get them?

You can make them yourself! I haven't played around with this much, but I think you can modify an existing realm structure or make one from scratch, and do a structure only export.