This is all based off my own time using this site so take what I say with a grain of salt.
Extra and Primary are the two types I personally use for curent partners. Each person can have one Primary partner and an umlimited number of Extras. If there isn't a Primary partner the next non-Ex relationship you add will be autimatically assigned as the primary partner, after that and non-Ex's will be Extras.
Lets say you have someone with multiple spuose all at once for whatever reason. whoever is considered the primary spouse you should set at the Primary partner and the others as Extras.
As you'll probably notice as you use this feture every relationship type not in someway related to marraiage will have the option for an end date. Not really sure what that's about, maybe I'm missing some kind of nuance.
