Refers to the way recording tape is wound onto a hub. The quality of the tape pack can significantly effect the performance of tape over time. With a good pack the tape is wound evenly so the edges form a smooth surface. If your tape winds onto a reel in a such a way that some of the edges stick out, those parts of the tape are much more likely to become damaged, especially over time. A good tape pack also implies that the tension of the winding is consistent and appropriate for the tape.