OK, we know the I-Form is a running formation and the Pro Set is balanced for running and passing. But these two use the same personnel; they just line up a little differently. Can anyone help me understand how GD processes these differently? Do you look for different player attributes? What practical difference does it make in your game plan?

"OK, we know the I-Form is a running formation and the Pro Set is balanced for running and passing."

We do?

Thats just how the game describes the formations.. but I really dont know how relevant it is now other then it gives you another option for your formation groups

I run both in an all pass offense in different worlds and have noticed no difference in performance at all.

Long time ago it was described by one of the Programmers. Slight advantage in passing towards Pro and same advantage in running for I.
But in my experience there IS no difference and anyone practicing both is wasting their points.

Agree with mojo. If you want to practice two formations, choose two that are much different. And get creative.