In the constructor for this object, first the code for the `TextEvent' command is registered as a command this object is interested in, and then all the words this object can understand (with their callback functions) are added to the object's internal list. Each time a new word is added to the VMD vocabulary, a new line must be added in this section to add the callback function (see the discussion of Command, section 6.6.1). The callback functions themselves should be declared in the Cmd*.h files, and defined in the Cmd*.C files.
When a text command is actually to be executed, is it broken into an argc, argv tokenized format, and given to the process_command routine. This searches for the first word in the internal list of known words, and calls the callback function with the argc, argv pair, and also the CommandQueue object to use for queuing new commands as well as the ID of this UIObject.
In the case where Tcl is being used, there is a slightly different order to these events. After a string is read, it is given to the Tcl evaluation routine, which checks it for special Tcl commands or for VMD commands. When VMD commands are seen, Tcl will tokenize the command itself and call a particular Tcl-tailored callback function. This callback function is registered with Tcl for every word in the VMD vocabulary. In this Tcl callback function, the first word is checked, and the regular text callback function is called.