The system fixture units is calculated in the non-blocking evaluation framework. The caller may set up callbacks that react to the asynchronous calculation results. If no callback is set up (e.g, called from third-party applications), the calculation is automatically switched to synchronous calculation so the caller can access the up-to-date result. Similarly, the public method get_ParameterValue(BuiltInParameter.RBS_PIPE_SYSTEM_FIXTURE_UNIT_PARAM) has the same behavior. Due to this change, the parameter RBS_PIPE_SYSTEM_FIXTURE_UNIT_PARAM no longer supports dynamic model update.