the quality that makes the Main Character uniquely qualified to solve the story's problem/achieve the goal
Just as a requirement defines the specific nature of things needed to achieve a particular goal, Unique Ability defines the specific quality needed to meet the requirement. Unique Ability is another way in which the Main Character is identified as the intersecting point between the Relationship Throughline and the Overall Story Throughline stories as it is only he who ultimately has what it takes to meet the test of the requirement and thereby achieve the goal. The Unique Ability need not be anything extraordinary but must be the one crucial quality required that is shared by no one else. Frequently, the Unique Ability is in keeping with the Main Character's position or profession, however it can be much more interesting to assign an incongruous Unique Ability. In either approach, it is essential to illustrate the existence of the Unique Ability in the Main Character several times throughout the story, even if it is not employed until the climax. In this way, it becomes integrated into the nature of the Main Character and does not seem conveniently tacked on when it is ultimately needed. Also, the Unique Ability can be extremely mundane. The key is that the ability does not have to be unique by nature, but just possessed uniquely in that specific story by the Main Character. Clever storytelling may arrange the climax of the story so that some completely ordinary and insignificant Unique Ability makes the difference in the outcome of a cosmic struggle.