The Subjective Story Concern describes the area of greatest conflict or divergence between the Main and Obstacle Characters. They might see eye-to-eye everywhere else, but when it comes to the Subjective Story Concern, they always come to blows. It is the nature of the way the thematic structure is created that the Concern of the Subjective Story Throughline will seem to grow out of the Main and Obstacle Concerns.
If the Subjective Story Concern were Obtaining, the Main and Obstacle would argue over whether or not they should have something. It might be something only one of them has or can have (who should have it?) or it might be something they must either have together or not at all.