According to Salvador Minuchin, the purpose of boundaries is to differentiate the subsystems and individuals within families so they can carry out their tasks without undue interference. Boundaries are essentially the rules which determine the membership in a subsystem and what role its members play. As such, individuals can belong to multiple subsystems which require specific skill sets and come with varying degrees of power. But without clear subsystem boundaries, a family will not function properly.
Families can be thought of as being on a continuum between two poles representing enmeshment and disengagement. Enmeshed families have diffused or blurred boundaries which mean they lack differentiation. Disengaged families, on the other hand, have overly rigid boundaries which prevent members of a subsystem from having much contact with those in other subsystems. While most families operate in the middle range, some operate at the extremes.
In the case of covert incest, it is common to find an enmeshed subsystem consisting of a parent (such as the mother) and a child, with the other parent (such as the father) completely disengaged from the parental subsystem. With no boundary surrounding the spousal subsystem, it is for all intents and purposes nonexistent.