Constitution is important in White Box combat because it determines your chance to survive adversity, most commonly encountered as your chance to survive going into negative hit points.
Those with a constitution of 13 or higher automatically survive adversity. For every point of constitution over 3 a character has, they have a 10% to survive, so 90% at 12, 80% at 11, 70% at 10, etc.
In the general course of combat, a character who reaches 0 hit points is incapacitated and unconscious. They are dead once they have negative hit points exceeding their level. A 0-level non-combatant is therefore dead at 0; a first level character must hit -2 before being dead (and cannot be immediately revived with bandaging, a cure spell or potion, etc.).
Once a character has gone under 0 hit points, however, they must roll to survive adversity when they attempt to restore themselves to a positive total, or the attempt fails and they die.
Tough characters, with a constitution 13-15, gain the benefit of remaining active and conscious until negative hit points exceed their level and are alive but unconscious until they exceed double their level.
Exceptionally tough characters, with constitution 16-18 (or fighting-men who have a constitution of 13 or over), get double the negative benefit, being active until they hit negative hit points two times their level, and being alive but unconscious until they exceed three times their level.
For example, a third level fighting-man will be:
|Unconscious from||-1 to -3||-4 to -6||-7 to -9|