The Dynamic Source function requires a little more context to understand what's going on...
The clue is, it is always the same module / module instance. So you don't have to create galleries for three, four, twelve or several dozens of galleries to display the corresponding images in each article. One module in dynamic mode is sufficient. Using the information in the custom field, OrielPro reads the corresponding image folder and shows you the images directly. This ensures that all galleries are displayed identically on all pages, and if you want to change something in the design you only have to do it in one place.