BootCamp/CSM/BIOS.
I'm starting the process of backing up my Mojave install from my hardware RAID dual SSD in order to wipe out entirely and reinstall Windows 10 in legacy BIOS as I want to completely redo the partition scheme into a more traditional arrangement that may hopefully alleviate some problems I'm experiencing (I had installed Windows 10 in front of OS X in the partitioning order under the impression Windows had to be in the first 2TB like Win XP/7, but now I'm reading that's no longer the case for Win10... or is that just for Win10 EFI???).
I'll keep Mojave in APFS for now to test out Win10 in legacy BIOS to see if can switch much more cleanly than presently where now I can't properly select Win10 to boot other than through the startup screen boot picker and selecting "EFI Boot"; very problematic with my RX580 which forces me to temporarily install my old Radeon 5780 everytime I want to use Win10 (my Win10 install crashes if using the GT120 by itself).