Usage relationships express dependencies between design parts, to show where design parts are re-used. To visually indicate that a design part is related by Usage, the design part icon shows a link: 
Design parts are organized into a product by defining relationships between them. Dimensions has two predefined types of relationship between design parts: breakdown and usage.
Breakdown relationships describe design parts that have a component relationship. The component design part can be thought of as a child, while the design part that includes it is the parent.
A child design part inherits the roles and permissions from its parent. This determines the roles users need to perform actions on items owned by the design part at different points in the process.
Usage relationships express dependencies between design parts, to show where design parts are re-used. To visually indicate that a design part is related by Usage, the design part icon shows a link:
A design part can have the following relationships with other Dimensions objects.
Items. Each design part owns a set of items associated with that part of the product, such as a functional specification document, a test specification, source code files, test data, and a class library.
Requests. A design part is related to the requests that affect the design part.
Baselines. A design part is related to the baselines that include the design part.
Related Topics
About Design Parts and Requests
Relating and Unrelating Objects