An add-in is a web-based application which can be associated with a chat room (see AddIn).
Assembly: Microsoft.Rtc.Collaboration.PersistentChat (in Microsoft.Rtc.Collaboration.PersistentChat.dll)
The chat room interacts with the add-in, sending it notification of inbound and outbound messages. The add-in also has access to metadata about the chat room and the messages which are posted to it.
An add-in must be registered with the system before it can be used on a chat room. The list of registered add-ins may be obtained from BeginGetAllAddIns(AsyncCallback, Object)