What does legacy mode refer to when dual booting an iMac?

I have a 2008 iMac that has MacOS and Linux Mint installed. When searching for a solution to a problem I have seen it stated that I need to boot in 'legacy' mode. What does this mean? Does it refer to old PC BIOS? Such a BIOS never existed on this iMac, it was always EFI. I installed using the Mint installer as far as I remember and I think it is using EFI. If I run efibootmgr I get the following: $ efibootmgr -v BootCurrent: 0000 Timeout: 5 seconds BootOrder: 0000 Boot0000* ubuntu HD(1,GPT,04202405-8589-4b15-b2c7-f72a95053a68,0x28,0x64000)/File(\EFI\ubuntu\shimx64.efi) Boot0081* Mac OS X PciRoot(0x0)/Pci(0x1f,0x2)/Sata(0,0,0)/HD(2,GPT,5f1185c2-ae3c-4590-99ba-c57db72f5442,0x64028,0xcbee240) BootFFFF* PciRoot(0x0)/Pci(0x1f,0x2)/Sata(0,0,0)/HD(2,GPT,5f1185c2-ae3c-4590-99ba-c57db72f5442,0x64028,0x79fc5c8)/File(\System\Library\CoreServices\boot.efi) I've seen references to rEFInd being used. This package is not installed. Can I just install that on the running system or do I need a clean installation? I'd prefer to keep the ability to run MacOS, though I don't normally need it. The problem I'm trying to resolve is waking the iMac/Mint from suspend, as described here: https://forums.linuxmint.com/viewtopic.php?t=335072