BPEL and similar languages have been provided a foundation for process-based implementation of composite Web services. These languages allow definition of orchestration processes without concrete binding information of involved partners. Although this approach facilitates customization of service discovery and selection before process execution, partner management during process execution has not been considered, such as rebinding of service providers upon events. In response to this problem, this study proposes a description model that facilitates runtime partner management in process-based services. The proposed model includes policy descriptions that allow insertion and customization of binding behavior according to surrounding environments and user preferences. The model also includes extensions in the standard process notation in order to allow process developers to give constraints to avoid adoption of inadequate policies as well as to define handlers to have common rebinding behavior reused. The proposed descriptions have been implemented and used in an agent framework for multimedia services.