Assembling a primitive having information elements with a structure recognizable by a terminal device and another entity, both of which communicate over a network

A data structure defining two-level identification allows the integration of mobile instant messaging to Internet based instant messaging, for instance, by providing an identification of both a user of the IM system (IM user) and an IM client used to access an IM system (IM client). The client may be a hardware device, software, or a combination thereof. A method, a terminal device with the client installed, a server and a system are shown for communicating such identification information between the terminal device and the server with a primitive having such two-level identification contained in information elements.