I presume they mean the standard hybrid ISO9660 LiveCD format that Linux distros tend to use. There's a lot of nuance there that I haven't touched in years but

I presume they mean the standard hybrid ISO9660 LiveCD format that Linux distros tend to use. There's a lot of nuance there that I haven't touched in years but needless to say between having multiple boot paths (I think like three--El Torito, BIOS boot sector, and UEFI) and very different media (CDROMs are, well, ROMs, thumb drives are writeable) there can be some complexity if you want to handle some more sophisticated portable OS stuff than just "run the OS installer". I'm sure this is particularly important for persistence, which can sort of be done with "traditional" live media but requires some special effort.