Is it used like in sort order ?
It is also used on the add media tool, and it is used to order the images when multiple images are added to a gedcom record.
So you have 5 images assigned to John Doe. With the order field you give each of them a number 1, 2, 3, 4, 5 for the order they should be listed in the gedcom record.
Thanks - I've never actually used the media tool, so I had no idea as to how it was to be understood.