The BPB is only the one in the bootsector of the volume.
The "double" BPB is the (modified) one in the (special) MBR.
Imagine that you boot to grub4dos from another device (let's say a USB stick) and try to access this "special" GPT disk.
If there is this "double" BPB in the MBR, than you can root to the device, i.e. root (hd0) just fine.
If there is not this "double" BPB in the MBR, than you cannot root to the device, i.e. root (hd0) will return Error 17: cannot mount selected partition, and if you try root (hd0 [TAB] grub4dos will try to autocomplete the line to (hd0,
What happens is that the grub4dos tries to read the volume BPB and cannot find it, with [TAB] it sees the (GPT) partition table, proceeds to read partitions in it and adds the comma.
When you are using the underfloppy, it is mapped "nowhere" unless you have the "double" BPB in the MBR, in which case it is mapped as a volume that starts at offset 0 on the device.
And sure, the "hidgpt63.mbr" already has the "double" BPB inside (it is hardcoded since the extents and the filesystem data of the underfloppy are known and fixed since the underfloppy image is also provided).
With the underfloppy you need the "double" BPB in the MBR to be able to access it as (hd0) or (fd0).
And you need to map the underfloppy extents in the GPT partition table if you want to access it as (hd0,0) or (fd0).
When you are using the GPT partition, it can be accessed "normally" via the GPT partition table as (hd0,0) or (fd0,0).
If you (optionally) add the "double" BPB to the MBR, it can be accessed also as (hd0) or (fd0), similarly to the underfloppy, it is also mapped as a volume that starts at offset 0 on the device.
In a nutshell:
1) with the underfloppy method you need (and already have) a "double" BPB in the MBR and you can optionally add the extents of the underfloppy to the GPT partition table.
2) with the partition method in theory you need not a "double" BPB in the MBR (it is optional) but seemingly in practice on your BIOS it is needed as well.