What is the difference between a union and an "isa" disjoint relationship? To me, it seems that a union and an "isa" disjoint relationship are the same. What is the difference between the two?
A isa usually indicates a subset. e.g. a male cat is a cat is a mammal. A union is the combination of two sets e.g. the union between cats and males is all cats and all males. Union is bidirectional whereas isa goes in one direction.