Only different races, nothing special. Since the majority of the NPCs are humans, only special NPCs are non-human such as NPCs Ao Yao, and Imp's (Nine Tailed) and others. The difference is that if there is a 'guanxi_unitIDa_unitIDb_RelationshipTypeID_ValueIntimacy' Command, we can immediately change their relationship with us into a Partner, if without the command (maybe) 100% we can't change the Relationship can only be limited to friends "non human"
But in my RelationshipTypeID "8 - Spouse" doesn't work, which can only be 5,6,9,10,11,7 (5 - Adop Daughter/Son), (6 - Adop Mother/Father), (9 - Partner), (10 - Apprentice), (11 - Master), (7 - Sworm Younger/Older Brother or Sister)