💚What does the idea of "separation" have to do with the Buddha's teaching?

What is meant with the idea of separation in the Buddhist interpretations of the Buddha's teaching & what does it have to do with the 3 marks of existence? What makes one person's karmic stream separate from another person's karmic stream? In accordance with the Buddha's teaching, what can we point to and say, that is where (any) two things are separate or not separate?