In Cross Summoner, usually units are determined by their elements such as Fire, Water and etc. But the units are also grouped depends on their affiliation. Affiliation are like clan or guild of the unit or where it belongs. A good example of that is the Royal Blood. Royal Blood separates themselves with other units with their imperial armors or knight-ish style.
What does Affiliation do?
Simple, some of the leader skills buffed up their own group instead of elemental base buff. For example, I have Elegant Spearman Schwarz who's leader skill affects Royal Blood affiliated units wherein it boost 60% of the unit's attack.
With this type of leader skill, you can mix different elements in one party instead of focusing on one element.