In other words, these are NOT "Genuine <fill in name of manufacturer> parts." The quality of aftermarket parts can vary widely, some engineered to be better than the originals, some poorly fitting and otherwise inferior crap. Caveat Emptor when buying aftermarket parts! But sometimes when the original parts are discontinued or the manufacturer has gone out of business, the aftermarket may be your only choice.
Quality Discount Press Parts and AAA Press are pretty much the only aftermarket suppliers of note in the flexo printing industry.