From /usr/share/X11/xkb/types/level5:

    type "EIGHT_LEVEL" {
	modifiers = Shift+LevelThree+LevelFive;
	map[None] = Level1;
	map[Shift] = Level2;
	map[LevelThree] = Level3;
	map[Shift+LevelThree] = Level4;
	map[LevelFive] = Level5;
	map[Shift+LevelFive] = Level6;
	map[LevelThree+LevelFive] = Level7;
	map[Shift+LevelThree+LevelFive] = Level8;
	level_name[Level1] = "Base";
	level_name[Level2] = "Shift";
	level_name[Level3] = "Alt Base";
	level_name[Level4] = "Shift Alt";
	level_name[Level5] = "X";
	level_name[Level6] = "X Shift";
	level_name[Level7] = "X Alt Base";
	level_name[Level8] = "X Shift Alt";
    };

To use the syntax is:

	key <AD05> { type[Group1] = "EIGHT_LEVEL", symbols[Group1] = [y, Y, f, F, a, a, a, a] };

And to use the level5 modifier, one can use setxbmap options (230208-1643 List of all xkbmap configuration options):

setxkbmap -option -option 'grp_led:caps' -option 'lv5:ralt_switch_lock' vv,ruua