The easiest way to distinguish the difference between these two types of relationships is based on the structure within a system or project. 

For a parent/child relationship, the child configuration item inherits the properties or parameters from its parent configuration item. The child may extend the properties of the parent for specific uses or provides additional clarity for what the parent is supposed to do. 

For a "used" relationship, the configuration item actively utilizes or references another specification. Unlike the child/parent relationship, these relationships can exist independently of each other. In other words, they are two independent entities that are working together in an interconnected system. 

Making this judgment, however, requires careful understanding of the context and the relationships between the configuration items involved.