The GtkStatusbar sets up an internal array of the messages that have been used, as they are being called. The context_id is the array index position that a given message holds.

As the message array is created on the fly, it is never the same twice running. The messages that are displayed most frequently during the status bar's lifetime will have low numbers, and a message that is never called will not be added to the array. It is far more efficient to use get_context_id() to generate a context id for a message than it is to allocate your own integer value.

The context_description is actually the text string used in the statusbar label.