You are correct, that particular property is not described in our documentation. We are constantly trying to improve our services and this is why I will make sure such description is introduced in the shortest period of time.
Till we update our documentation you can take a look at the attached project. It demonstrates how the RadCalendar.AppointmentSource
property can be utilized. If you need to further customize any cells of the calendar you can refer to our Display Custom Event Information in Calendar Cells
I hope this information will help you achieve your requirements. Feel free to contact us if any fuhrer assistance is needed.
Pavel R. Pavlov