For example, the duration of operations is a Quality of Service (QoS) parameter. In the level of classes, the QoS parameters include these attributes among others:
Values for these parameters are needed to perform schedulability analysis in both the empirical and theoretical ways.
One important goal of future real-time extensions to the UML is to identify an appropriate set of QoS timeliness properties. The natural mechanism to do that would be UML-tagged values.
IBM® Rational® Rhapsody® has an extensible property mechanism that closely corresponds to the notion of UML-tagged values.