This is to say that the act of drawing the boundary itself provides the basis for saying that one person is separate from another psychologically, but does so only by drawing a distinction between those two people, which implies a relationship, never the less.
Self cannot exist without also "Not-self" existing, just as figure cannot exist without ground against which to contrast.
Social groups of any size are seldom uniform things.
Rather, there are frequently sub-groups that form within larger groups that have special status and power within the group as a whole.
The prototype for this sort of power hierarchy is the nuclear family (e.g., parents with children).
Parents function as a powerful and bounded subgroup within the larger group known as the family.
The term originated within the field of Biology when the study of living systems such as oceans, forests and prairies revealed how inseparably interlinked many species are. Flowers require bees to pollinate for them so that they can reproduce, while bees require flower pollen for food, or whatever it is that they do with pollen.
While a bee and a flower can be said to exist independently of one another, they do not occur that way in nature, and neither might survive for long if indefinitely deprived of the other.
In this very social vision of therapy, groups of people operating as units are the proper client to which therapists must address their efforts.
Individuals exist, but problems they experience are not individual but rather are social in nature.
Each person can be said to have a psychological identity boundary around themselves by which they distinguish themselves from other people.
Like other boundaries, this identity boundary both separates people and also defines how they are linked together.
Younger children function as a subgroup as well, but one with less power than parents have.