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 property b on class A is a subset of the property part on class B. This means that the elements associated with A through b are contained within the larger set associated with B through part. 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.