How is default boot image maintained internally?

[Brett Russ]

I am curious as to how GRUB remembers which image is to be booted next 
if I execute a 'savedefault ... -once' command from the shell prior to a 
reboot.  It isn't stored in the .conf/.lst files that I can tell.  I am 
hoping no writes take place to the MBR or the partition boot sectors to 
store this info.

Is it also true that there is no way to specify a text label (other than 
"saved") as the target for the default command?

Thank you,
Brett