If this variable is set, it identifies a menu entry that should be selected by default, possibly after a timeout (see timeout). The entry may be identified by number or by title.
If the entry is in a submenu, then it must be identified using the titles of each of the submenus starting from the top level followed by the number or title of the menu entry itself, separated by ‘>’. For example, take the following menu structure:
Submenu 1 Menu Entry 1 Menu Entry 2 Submenu 2 Submenu 3 Menu Entry 3 Menu Entry 4 Menu Entry 5
“Menu Entry 3” would then be identified as ‘Submenu 2>Submenu 3>Menu Entry 3’.
This variable is often set by ‘GRUB_DEFAULT’ (see Simple configuration), grub-set-default, or grub-reboot.