Cels are what contains all the information that makes your Kiss unique. All the objects in the Kiss are stored as cels.
Every thing has to be stored as a separate cel, including different layers of the same item. If I don't take layers into consideration, my doll will look like this:
But if I use layers, she will look like this instead:
That is because the arm is a separate cel, defined above the skirt.
When you have an item with multiple layers, you have to make sure all the cels line up. the top left corner on each cel has to be the same place in relation to the entire item. So the arm has a lot of blank space (in the transparent color) above it so it will line up with the body.
You can have as many layers to an item as you wish. You will combine them in the cnf file so they are all attached.