For early QoS analyses, domain experts assist system architects in specifying the user interaction with the system. As they are familiar with the business domain and the targeted end-users, they should best be able to specify the anticipated usage scenarios and workloads of the system. The usage specifications may be based on experiences with similar legacy systems or on market analyses of the business domain. In the PCM, the usage specification consists of usage models (see Section 3.5.2), which are similar to UML use cases with attached UML activities. They additionally contain stochastical information (e.g., probabilities of choosing a branch in an alternative) and the notion of workload to characterise the number of users in the system, which is especially relevant for performance predictions. Usage models may be refined with a parameter model (explained in Section 3.5.3) to characterise the data values passed to component services by users.