In photography, when you copy a negative you get a positive. When you copy a positive, you get a negative.
So you have an original negative. You make a copy, which is a positive; this is the IP. But you can't make prints off of this, because they will be negative. You copy it, making the IN, and then from this negative you can now make positive prints for projection. The result is that they are copies of a copy (IN) of a copy (IP) of the original.
Does this make sense? It is indeed a bit convoluted.