If one were to use the coordinate system defined for the first image, positive X would now be at the left, positive Y to the right and so on. This could become quite confusing. To avoid this problem, each image is analyzed using its own local coordinate frame so that X is always to the right, Y is always to the left, etc. By using this approach, the user is always easily oriented and can think the same way for each image. The local frame is used when analyzing images using single image techniques.
Dimension allows the user to analyze multiple images simultaneously.
The common coordinate frame dispenses with the local frames in favor of a single frame that is consistent from image to image. Using the common frame ensures that the same point on an object has the same X, Y, Z coordinates in each image. The multi-image adjustment techniques could not work otherwise. The common frame is computed automatically. The OFT converts the different local coordinate frames into a common frame for all images involved in the analysis.