The intent of this regulation is to help the radiologic technologist optimally position the AEC detector. Under some AEC detector designs it may be difficult to show all possible positions. Some detectors cover the entire area of the image receptor and once the exposure begins, they automatically select the region of maximum density as the active area. For these systems, indication of the entire potential active area, along with appropriate instructions (usually found in the user manual), would satisfy this requirement. Since the area is automatically selected, the display of the size of the detector is not required. Other designs may have an essentially infinite number of locations under all or part of the image receptor. An indication of the complete range and detector size, coupled with adequate instructions, would be sufficient. Still others may indicate the range of multiple positions on the paddle. Again, this would satisfy the requirement. There may be other methods employed that also satisfy the requirement. The key is that the operators know what areas they may select and the size of the detector.