Ok I've made an example for you since I think you don't quite get it.
http://img36.imageshack.us/img36/8007/captureike.jpg
When you open a .dvd file up in notepad, a screen like that shows up. It will display the name of the .iso file and below that it will say layerbreak=(the layerbreak for the .iso, which for the 360 is 1913760). Since it displays the name of the .iso, it will only work with that .iso.
Technically, you can use the same .dvd file for other .iso files, by opening it in notepad and editing it.
I hope someone can explain it better than I can, I'm just trying to educate you here.