Subsets   A value of subsets specified for the property string of an association end indicates that the collection does not contain all the available instances defined by the named property. This is an application of the mathematical concept of subset, which means that every instance at the association end marked as subsets, must exist in the parent property.

Explanation

When a class (or any other classifier) is specialized, properties are redefined and are available in the specialized (child) class. It is common that we may wish to restrict the number and nature of the collection of instances at the association end of the specialized class. This concept exists in our daily lives; where we may have a group of people we call acquaintances we may have a subset of this group we think of as friends. Our group of friends is a subset of our acquaintances with the following restrictions, every friend is an acquaintance (but not every acquaintance is necessarily a friend), and the number of friends is never greater than the number of acquaintances.