BootOrder is one of the UEFI boot variables, stored in NVRAM along with other firmware configuration (what used to be "BIOS settings").
In Linux, you can use
efibootmgr -v to view the boot variables.
# efibootmgr -v
Timeout: 6 seconds
Boot0000* debian HD(1,GPT,4fb8aadb-9507-44b5-8cab-a052a0091e2b,0x800,0x100000)/File(\EFI\DEBIAN\GRUBX64.EFI)
Boot0001* rEFInd Boot Manager HD(1,GPT,4fb8aadb-9507-44b5-8cab-a052a0091e2b,0x800,0x100000)/File(\EFI\REFIND\REFIND_X64.EFI)
Boot0002* UEFI: IP4 Realtek PCIe GBE Family Controller PciRoot(0x0)/Pci(0x1c,0x2)/Pci(0x0,0x0)/MAC(107b447ad398,0)/IPv4(0.0.0.0:0<->0.0.0.0:0,0,0)..BO
Boot0003 UEFI Shell HD(1,GPT,4fb8aadb-9507-44b5-8cab-a052a0091e2b,0x800,0x100000)/File(\EFI\tools\shell.efi)
Boot0004* UEFI:CD/DVD Drive BBS(129,,0x0)
Boot0005* UEFI:Removable Device BBS(130,,0x0)
Boot0006* UEFI:Network Device BBS(131,,0x0)
Here I have a Debian 9 system, with two bootloaders installed: the Debian default GRUB, and the more visually-oriented rEFInd, which I've set as the primary bootloader. I also have the UEFI Shell as a boot option, although that option is currently disabled.
Note that the BootOrder lists the order in which the other BootNNNN options will be tried.
You might have a BootNNNN line that's left over from the Windows or Arch installation. Since the bootloader file it refers to no longer exists, it is going to be useless. You can use
efibootmgr -B -b NNNN to delete it: just replace NNNN with the actual BootNNNN number you wish to delete.