Outcome is the difference between ActionResult and DefenseValue. It can be positive, zero, or negative.
A positive Outcome generally means that the stunt or action succeeded. In most cases, the Outcome is used to determine how successful it was - for example, it is added to Damage of an attack and used to determine the degree of success on interactionStunts.
An Outcome of zero means the stunt was just barelysuccessful, and will do base Damage or otherwise have a minimal result.
A negative Outcome means the stunt failed. In most cases, that is enough, as failed stunts have no additional effect. In certain cases the negative outcome has an effect, like in a DeathTest.