In UML, the{subsets}keyword is used to indicate that one property is a subset of another property. In the provided diagram fragment, the association end-b {subsets part}indicates that the propertybon classAis a subset of the propertyparton classB. This means that the elements associated withAthroughbare contained within the larger set associated withBthroughpart. This kind of relationship is typically used in more complex class models where a part-whole hierarchy is represented, and it allows the model to show that certain associations are more specific subsets of a more general association. This usage is explained in the UML 2.x Superstructure Specification, where the semantics of association subsets are defined.