A high density pack refers to the buffing wheels being constructed with a larger amount of fabric. Normally, buffing wheels are constructing using an iris machine where fabric is wrapped around a round barrel or drum. The fabric is wrapped around this drum half the amount of the total finished buffing wheel's ply count. A high density pack, will use a larger drum than the standard, creating a greater amount of fabric per buffing wheel and a tighter pack. This provides superior life but can also produce a slightly stronger cut, which is why you'll commonly see this type of construction only on the buffing wheels used for an initial or secondary cut.