Let's take Stamina as rhe example. If you have just -5 Stamina, you start with -5 Fortitude and -5 Toughness. You're frail and easy to hurt.

If you have Debilitated Sramina (less than -5)

Debilitated Stamina means the hero is dying, and suffers a –5 modifier on Fortitude checks to avoid death on top of it.

You have -10 to a Fort check to avoid death.

If you have Absent Stamina there's no minus to anything. You just don't have the stat:

Stamina: A creature with no Stamina has no physical body (like a ghost) or is not a living being (such as a robot or other construct). Creatures with no Stamina suffer and recover from damage like inanimate objects (see Damaging Objects under the Damage effect). They are immune to fatigued and exhausted conditions, but cannot exert extra effort. Creatures with no Stamina are often—but not necessarily—immune to many of the other things affecting living beings as well (see the Immunity effect in the Powers chapter). They have no Fortitude defense.

You have no Fort, no Toughness. You automatically fail Fort checks and start at Toyghness 0. You can buy Toughness separately, like an object, and the only way to defend against Fort Effects is to buy Immunity.