If you have an iMac (27-inch, Late 2012) with a 3TB hard drive, you must remove its Boot Camp partition before you can install macOS Mojave 10.14. After you upgrade to macOS Mojave, you won’t be able to use Boot Camp to install Windows on this Mac.
If you install macOS Mojave on this iMac, you won’t be able to use Boot Camp to install Windows.
I’d love to know the story behind this one.