DIPPING:

Most professional breeders ask for dipped onionsets because of the high disease pressure nowadays. Dipping is putting the sets in a bath of water with added chemicals. This sees to the bulb getting a protecting shield around the skin which defends the set that it will get certain deseases which it would have had, when the sets would not have been dipped.

In a lot of areas it is impossible for the professional breeders to grow onions before the sets first having dipped. Therefore dipping is a necessity to prevent whiterot.

"MAAS" dips onionsets in the bag, because then the chemicals remain better on the bulb. Therefore the risk for diseases is smaller than when the bulbs are first dipped and later on packed in bags.
When the sets are packed after dipping a number of onionsets loose part of their skin which means loss of protecting skin.