hd-media and uefi

Can someone point me to a spec or something that defines waht a valid uefi
boot device is?

background: I use the hd-media image (becuase it is a fat fs so I can
mount and edit kernel boot parameters) but hd-image is not efi at all (it
doesn't even try.)  I have an MR that addresses it but needs work:
https://salsa.debian.org/installer-team/debian-installer/-/merge_requests/7

The post is related, but new as I just now found an HP server that won't
boot my patched hd-media.   and I am compelled to figure out what correct
is.

What I did figure out: fdisk /dev/usb-stick, create 1 partition.  dd the
hd-media.img to the partition instead of the host device.   The HP likes
this.

Once I find a good description of what the efi spec is, we can talk about
making hd-media efi.