In contrast to computer systems, human behaviour is only predictable to a certain degree of probability. In semi-automated business processes human actors are guided by predetermined policies and regulations but retain the freedom to react to unforeseen events. For example, if an urgent prescription has to be dispensed by a pharmacist and the current pharmacist on duty is busy, it is likely that they would interrupt their current activity. The assignment policies in a model of this process should accurately define exception handling procedures in order to realistically simulate business processes. Other policies may require task assignment to the least-qualified person available to do the job if the people involved have different levels of access rights and qualifications.