I have some of these sets. They come with a sheet of white Ejection Foam, which you peel off individual squares of and stick over the cutting side of the die, i.e. the actual letter shape.It will cover the whole surface, covering up the letter shape,but when you cut with it, it cuts perfectly.
I've scanned in a sheet of the foams and what they look like on the dies to help you see what they will look like.
If they don't look like this, then they are the clear shims, sorry I don't have any of those left I can scan
The clear sheet are shims, which you can stick onto the reverse side of the die if it fails to cut very well, which sometimes happens.
Sometimes one edge or corner doesn't cut cleanly. You can also overcome this by turning the die around.Or you can use a thin piece of copy paper as a shim. Are you using the Squeeze tool, or cutting with a 'Bug or other machine?