As stated on page 322, a Character risks the Unconscious State when they have zero Body, Mind or Soul. Every Round, they have to make a Will Check to stay Conscious and perform Secondary Actions.
If a Character has less than zero in Body, Mind or Soul, then they are Dying (page 324). Anyone who is Dying also falls under the rules for Unconsciousness. Every Round, they must make a Will Check to stay Conscious.
This was written this way for clarity. Zero is Unconscious. Less than zero is Dying and Unconscious.