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When an appointment is saved with multiple resources, unique appointments for each resource will be saved with its id. Therefore, when an appointment is opened, only its respective resource will be shown in the appointment window which is the default behavior. Currently, there is no option to display all the resources for the multiple resource appointments in the appointment window. In our latest release version (18.104.22.168) when an appointment is saved/edited with multiple resource (say 2), parameters shown below will be retrieved which helps to keep track the appointment added for the resource. We have prepared a sample for your reference which can be download from the below location.
1. BeforeAppointmentChange event parameters
2. BeforeAppointmentCreate event parameters