How does 3D Slicer coordinates work?

I have two 3D images. One image is much bigger because the other image is cropped from the bigger image. Let’s say I random crop a 50x50x50 image from the 100x100x100. How would I be able to know the coordinates of the smaller image (inside the bigger image) using python when I import both of those?

I tried using the RAS method but it seems that the coordinates I am getting are wrong.

Thank you!

This should be a good place to start: