Windows Boot Manager can chainload into any arbitrary bit of code if you point it where it needs to hand off.
It's a feature that goes back to Windows NT (NTLDR) supporting dual boot for Windows 9x, but it can be repurposed to boot anything you would like so long as it can execute on its own merit.
eg: Boot into Windows Boot Manager and, instead of booting Windows, it can hand off control to GRUB or systemd-boot to boot Linux.
>Windows Boot Manager can chainload into any arbitrary bit of code if you point it where it needs to hand off.
With the NT6 bootloader this appears to be limited to operating only in BIOS mode using bootmgr.exe. The traditional chainloading is still possible by pointing to a binary file which is a copy of a valid partition bootsector, whether it is a Microsoft bootsector or not.
The equalent BCD for UEFI mode uses bootmgr.efi (instead of bootmgr.exe), and does not seem to be capable of chainloading even when there is an equivalent BOOTSECTOR bootentry on the NT6 multiboot menu.
It would be good to see an example of the NT6 bootloader successfully handling UEFI multibooting which includes starting Linux from the EXTx partition it is installed on. Still works perfectly in BIOS since early NT, but in UEFI not so much.
Windows Boot Manager can chainload into any arbitrary bit of code if you point it where it needs to hand off.
It's a feature that goes back to Windows NT (NTLDR) supporting dual boot for Windows 9x, but it can be repurposed to boot anything you would like so long as it can execute on its own merit.
eg: Boot into Windows Boot Manager and, instead of booting Windows, it can hand off control to GRUB or systemd-boot to boot Linux.