When opening an image, the application tries to discover other files, such that all selected files follow a common naming scheme.
It can happen that a file can belong to multiple potential naming schemes, e.g. "img_01_32.png" may belong to the set "img_00_32.png, img_01_32.png, img_02_32.png,..." or it might belong to "img_01_31.png, img_01_32.png, img_01_33.png,...".
If the application finds that the set membership of a file is unambigious, it asks the user (in a dialog) to choose one.