What are the basic problems of photogrammetry coordinate system?

Look at this picture, it should be relatively oriented or meet in front. In fact, I was confused when I read here, but I can tell you responsibly that not knowing here has no effect on your study of photogrammetry. According to my own understanding, I still think that X is the direction of the route. Ideally, the direction of B is parallel to X, but it is unrealistic to have a plane rotation.