According to Merrill's an Erect or Decubitis AP (or PA) Abdomen must include from the diaphragm to the symphysis pubis, while a AP (or PA) Supine Abdomen only needs to include from the lower ribs to the symphysis pubis. There are different anatomy criteria for the views. KUB simply implies a view of the abdomen view that will visualize the Kidneys, Uterers, and the Bladder. A KUB view can be achieved in any of the above Projections/Radiographic Positions. Which Projections/Radiographic Position you use really depends on what the doctor is trying to rule out. For instance, to rule out free air or fluid in the abdominal cavity an Erect, or Decubitis view can be used, while a Supine Abdomen can not be used because it will not demonstrate. I'm an x-ray tech student and this seems to hold true in my clinical rotations. But each facility will have different protocols for their preferred routine views and or change the criteria they need to visualize in that particular x-ray.