Sets the volume that's used for subsequent file accesses (by icon and loader, and by implication by initrd if loader follows volume). You pass this token a filesystem's label, a partition's label, a partition's GUID, or a volume number. A filesystem or partition label is typically displayed under the volume's icon in file managers and rEFInd displays it on its menu at the end of the boot prompt string. If this label isn't unique, the first volume with the specified label is used. The matching is nominally case-insensitive, but on some EFIs it's case-sensitive. If a filesystem has no label, you can use a partition GUID number. You can also use a volume number followed by a colon, such as 0: to refer to the first filesystem or 1: to refer to the second. The assignment of numbers is arbitrary and may not be consistent across boots, though. It might change if you insert an optical disc or plug in a USB flash drive, for instance. If this option is not set, the volume defaults to the one from which rEFInd launched.
It’s possible I read too much into it and assumed that the “aribtrary” part had to do with the boot order in BIOS, maybe it has more to do with how refind interprets it instead?