Final answer:
The AP oblique RPO/LPO SI joint refers to the position used for imaging the sacroiliac joint when the patient is lying on their back. The joint is formed by the sacrum and the ilium bones of the hip.
Step-by-step explanation:
The AP oblique RPO/LPO SI joint refers to the position of the sacroiliac joint when the patient is lying on their back with the X-ray beam angled from the posterior to the anterior direction or vice versa. This position allows for a better visualization of the joint. The sacroiliac joint is formed by the articulation of the sacrum and the ilium bones of the hip.