Coil set is when the col of metal takes on a curvature that follows the way that the coil has been wound. In our industry (small metal parts), we define coil set as when the metal strip retains some of the curvature after unwinding - like a piece of paper retains a curl after you roll it up. Negative coil set is where the metal does the opposite - the strip has a curl opposite to the curvature of the coil. This is generally due to the shearing forces acting on the metal strip at the time of slitting down tot he width that you want. We regularly have issues with metal strip which is "up sheared" - i.e. the cutting knives are forcing that edge upwards. Alternate strips on a slit-down reel of metal will have an element of negative coil set; if it doesn't affect your porcess then it's OK. The problems start when negative coil set means that the metal lifts up and jams in the tooling - then it's too much!
Hope that helps!