I don't think he measn the inner and outer wheel seals.
He means the roller bearings some are open cages and some are "sealed" with a dust shield of rubber (RS) or a metal plate (ZZ)
I personally like to get the ones with the rubber shield RS. And then I pry the side seals off of each bearing, pack the with grease, then reinstall one dust shield on each bearing.
Install them with the open sides facing in towards the spacer.
I fill that spacer area between the 2 bearings with about 30% with grease. Too full is no good, but you want some grease to spread when it gets hot int here.
still use the factory wheels seals inner and outer but the dust shield in the bearing helps as a secondry barrier.