The idea of segmental relationships is seen most commonly in urban sociology. It is used by sociologists who believe that urbanization has essentially weakened the ties that exist between people in a society. They argue that relationships between city dwellers are more segmental than those between people in small towns. The idea of the segmental relationship holds that people in cities tend to interact with one another in only one way.
In small towns and rural areas (which dominated our lives up until the last century or so), people had multiple types of relationships with one another. My next door neighbor might also be my coworker. We might go to the same church. We might even be related to one another in some way. This gives us multiple points of interaction with one another.
Some sociologists say that in cities, by contrast, we tend to have only one sort of a relationship with most people that we meet. We know people only from work or only from church or only because they are our neighbors. This is a segmental relationship. Each relationship we have with another person is segmental because it is based on only one thing. This means that we have fewer connections to other people and that, therefore, the ties that bind city dwellers together are weaker than those that bound people who lived in rural areas and small towns.