When does a grub2 loopback need to know the partition that contains the file?

[Parke]

Hello,

When does a grub2 loopback need to know the partition that contains the
file?

Consider the following grub2 menu entry:

    menuentry "SliTaz Rolling 20181204" {
      set isofile="/home/[USER]/slitaz-rolling.iso"
      loopback loop $isofile
      linux (loop)/boot/bzImage lang=en kmap=us autologin isofrom=$isofile
boot=live noeject noprompt root=/dev/null
      initrd (loop)/boot/rootfs4.gz (loop)/boot/rootfs3.gz
(loop)/boot/rootfs2.gz (loop)/boot/rootfs1.gz
    }

Sometimes the above entry will work, but other times I need to specify the
disk and partition that contain the `.iso` file.  For example, sometimes I
need to change the `set isofile` line to something like the following:

      set isofile="(hd0,gpt4)/home/[USER]/slitaz-rolling.iso"

When is it required to specify the disk and partition?  Are there some
situations where grub will scan multiple disks and partitions?  Or is there
just one "default" disk+partition, and any other partition will need to be
manually specified?  Is this behavior documented somewhere?

Some possibly relevant factors:  Filesystem (specifically, btrfs vs
ext2/3/4).  Whether the partition is the "root" partition.

For reference:
https://www.gnu.org/software/grub/manual/grub/grub.html#loopback

Thanks in advance!

Parke