When a thing connects to the IoT Portal it establishes a session with a unique session ID. In case of bound things, a proxy acts as an intermediate thing which acts as a virtual representation of all the things connected through it. Bound things are connected to the IOT portal through an intermediate thing (proxy) that has established a direct connection to the IOT portal. Bound things share the same session (and hence session ID) of the intermediate thing they are bound to. When the proxy establishes a connection with the IoT portal all things that are bound to the proxy thing will receive data from the IoT portal through the proxy's connection. The Proxy initiates the session binding by sending a thing.bind to bind the things to the current session. For example, in the diagram below when a mail is sent to Thing A (using either method.exec or mailbox.send), a session gets established with a valid session ID. Using this session the IoT Portal sends the mail to Thing Y (Proxy) which is a virtual representation of Thing A, Thing B, and Thing C. Thing Y then delivers the mail to Thing A using an established communication path.
A proxy thing communicates with the IoT Portal instead of the bound things since the bound things do not use the same protocol for communication. Hence the proxy is used as a translator and sends the thing.bind command to bind the things.
The Bound things tab is available for a thing that is a Proxy and it lists all the things that are bounded to it.