That charges two 3s batteries in series... To charge in series your batteries much be discharged exactly the same, they must be pretty much identical both in specs and in state of charge. So if you have two identical 3s 2200s that you fly in series (to make a 6s pack) should come down exactly the same. They can be charged in series.
For parallel charging they just need to be similar. Two 2200mah 3s lipos of different brands that were flown in different aircraft can be parallel charged together. Of course the packs from the other example can be charged in parallel as well. So why not just parallel charge?